btrfs: initialize inode mapping flags for cached inodes

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

btrfs: initialize inode mapping flags for cached inodes

[BUG]
When running generic/795 with 8K block size, 4K page size, the test
always fails, triggering some ASSERT()s related to folio size:

795 (241074): drop_caches: 3
assertion failed: IS_ALIGNED(start, blocksize) && IS_ALIGNED(end + 1, blocksize), in extent_io.c:1404 (blocksize=8192 root=262 ino=258 start=16826368 end=16830463 mapping min order=0)
------------[ cut here ]------------
kernel BUG at extent_io.c:1404!
Oops: invalid opcode: 0000 [#1] SMP
CPU: 8 UID: 0 PID: 241105 Comm: fsstress Tainted: G OE 7.2.0-rc5-custom+ #442 PREEMPT(full) f4bfb352566f3949f29c233ce6f735050a03b245
Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
RIP: 0010:assert_folio_range.cold+0x3d/0x3f [btrfs]
Call Trace:
<TASK>
btrfs_read_folio+0x9e/0x170 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3]
prepare_one_folio.constprop.0+0x104/0x2a0 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3]
btrfs_buffered_write+0x285/0xa50 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3]
btrfs_do_write_iter+0x1aa/0x210 [btrfs 4cd1dd93b341b8ef766643f9512f4a86259567a3]
iter_file_splice_write+0x31a/0x540
direct_splice_actor+0x53/0x170
splice_direct_to_actor+0xe9/0x240
do_splice_direct+0x76/0xb0
vfs_copy_file_range+0x1fd/0x630
__x64_sys_copy_file_range+0xf9/0x220
do_syscall_64+0xe1/0x790
entry_SYSCALL_64_after_hwframe+0x4b/0x53
</TASK>
---[ end trace 0000000000000000 ]---

The ASSERT() itself is added by a later patch.
The crash is triggered with that new debug patch, and without this fix.

[CAUSE]
In the above case, the start 16826368 is properly 8K aligned, but the
end (16830463 + 1) is not 8K aligned.
Furthermore the mapping's minimal folio order is 0, not the expected 1
for 8K block size with 4K page size.

So this means some inodes do not have btrfs_set_inode_mapping_order()
called on it.

The missing btrfs_set_inode_mapping_order() call happens for cached
inodes, through the following events:

- btrfs_create_new_inode() called for inode X
Which properly sets minimal folio order for the VFS inode.

- btrfs_update_inode() called for inode X
Which calls btrfs_delayed_update_inode() to create a delayed_node
into root->delayed_nodes xarray.

- Drop cache/memory pressure, evicting in-memory inode X
Which evicted the inode X, but delayed_node is still in
root->delayed_nodes for future reuse.

- btrfs_iget() for inode X called again

btrfs_iget()
|- btrfs_iget_locked()
| |- iget5_locked_rcu()
| Which creates a new vfs_inode for btrfs, whose mapping still
| has the minimal order as 0.
|
|- btrfs_read_locked_inode()
|- btrfs_fill_inode()
| |- btrfs_get_delayed_node()
| Which found out the previous node, and use that delayed
| node to initialize the new inode.
|
|- filled = true;
|- if (filled) goto cache_index;
Which skips the btrfs_update_inode_mapping_flags() and
btrfs_set_inode_mapping_order() calls.
So the inode still has minimal folio order set as 0, not
the required 1.

Thus later page cache read will get a folio whose size is smaller than
block size, as the mapping has its minimal folio order set as 0 not 1,
then trigger the ASSERT().

[FIX]
Move the btrfs_update_inode_mapping_flags() and
btrfs_set_inode_mapping_order() calls under cache_index label,
so that the mapping flags and minimal folio order is always set
no matter if we have a cached inode.