xfs: fix under-reservation of blocks when repairing sf directories

In the Linux kernel, the following vulnerability has been resolved:

xfs: fix under-reservation of blocks when repairing sf directories

Whilst running QA on XFS for-next as of 7.3-rc2 with MKFS_OPTIONS="-n
size=8192", I observed the following (trimmed) dmesg splat:

XFS: Assertion failed: args->total >= dp->i_nblocks - nblks, file: fs/xfs/libxfs/xfs_da_btree.c, line: 2387
WARNING: fs/xfs/xfs_message.c:104 at assfail+0x46/0x4a [xfs], CPU#0: xfs_scrub/1426511
CPU: 0 UID: 0 PID: 1426511 Comm: xfs_scrub Tainted: G W 7.3.0-rc2-djwx #rc2 PREEMPT(lazy) 6e418570b606a39783b0e7e7b30dc407b965f9e8
Tainted: [W]=WARN
RIP: 0010:assfail+0x46/0x4a [xfs]
RSP: 0018:ffffc900010d7890 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000000ffffffd1
RDX: 0000000000000000 RSI: 0000000000000021 RDI: ffffffffa059fd38
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000000000a R11: 000000007fffffff R12: ffffc900010d7940
R13: ffff888368d8f980 R14: ffffc900010d7a48 R15: ffffc900010d78d0
FS: 00007f445c5ce680(0000) GS:ffff8884a97ea000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f443803b9a8 CR3: 0000000107a4b000 CR4: 00000000003506f0
Call Trace:
<TASK>
xfs_da_grow_inode_int+0x2e0/0x300 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xfs_dir2_grow_inode+0x6e/0x150 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xfs_dir2_sf_to_block+0x149/0x870 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xrep_dir_swap_prep+0xe2/0x110 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xrep_dir_swap+0xfb/0x2f0 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xrep_dir_rebuild_tree+0x99/0x100 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xrep_directory+0x83/0x1c0 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xrep_attempt+0x4f/0x1e0 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xfs_scrub_metadata+0x393/0x5b0 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xfs_ioc_scrubv_metadata+0x306/0x570 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
xfs_file_ioctl+0xa4f/0x1150 [xfs 5de2257e14108c136f11317e6bbb8ac77efd392c]
__x64_sys_ioctl+0x76/0xc0
do_syscall_64+0x7a/0x3b0
entry_SYSCALL_64_after_hwframe+0x4b/0x53

This is a consequence of commit 0fe77e57588b98, which added the
following assertion to xfs_da_grow_inode_int:

ASSERT(args->total >= dp->i_nblocks - nblks);

Tracing this back to xrep_dir_swap_prep, I noticed that the xfs_da_args
object that's passed to xfs_dir2_sf_to_block sets args->total to 1.
This is incorrect because mkfs set the directory block size to 8k and
the filesystem block size to 4k. In other words, args->total should be
2 here, not 1.

Dave Chinner tripped over the same problem with the same branch through
a different channel -- his test setup set the fs block size to 1k, in
which case the directory block size is still set to 4k. Here,
args->total should be 4.

Changing the assignment of args->total to sc->mp->m_dir_geo->fsbcount
makes the assertion go away, but that isn't a complete fix. In
xrep_tempexch_estimate, we also incorrectly assume that a shortform
conversion requires 1 fsblock when it should be m_dir_geo->fsbcount.
Without that, we can under-reserve space in the transaction and cause a
filesystem shutdown.

Note that the xfs_dabuf_nfsb helper will compute the correct value for
directories and xattr, so we use that instead of open-coding the logic.
Also fix xrep_xattr_swap_prep to assign args->total via xfs_dabuf_nfsb
to avoid one logic bomb if we ever support multi-fsblock attrs.

Tripped-by: 0fe77e57588b98 ("xfs: assert the reservation covers each da fork growth")