btrfs: fix resolving backrefs for inline extent followed by prealloc

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

btrfs: fix resolving backrefs for inline extent followed by prealloc

If a file consists of an inline extent followed by a regular or prealloc
extent, then a legitimate attempt to resolve a logical address in the
non-inline region will result in add_all_parents reading the invalid
offset field of the inline extent. If the inline extent item is placed
in the leaf eb s.t. it is the first item, attempting to access the
offset field will not only be meaningless, it will go past the end of
the eb and cause this panic:

[17.626048] BTRFS warning (device dm-2): bad eb member end: ptr 0x3fd4 start 30834688 member offset 16377 size 8
[17.631693] general protection fault, probably for non-canonical address 0x5088000000000: 0000 [#1] SMP PTI
[17.635041] CPU: 2 PID: 1267 Comm: btrfs Not tainted 5.12.0-07246-g75175d5adc74-dirty #199
[17.637969] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[17.641995] RIP: 0010:btrfs_get_64+0xe7/0x110
[17.649890] RSP: 0018:ffffc90001f73a08 EFLAGS: 00010202
[17.651652] RAX: 0000000000000001 RBX: ffff88810c42d000 RCX: 0000000000000000
[17.653921] RDX: 0005088000000000 RSI: ffffc90001f73a0f RDI: 0000000000000001
[17.656174] RBP: 0000000000000ff9 R08: 0000000000000007 R09: c0000000fffeffff
[17.658441] R10: ffffc90001f73790 R11: ffffc90001f73788 R12: ffff888106afe918
[17.661070] R13: 0000000000003fd4 R14: 0000000000003f6f R15: cdcdcdcdcdcdcdcd
[17.663617] FS: 00007f64e7627d80(0000) GS:ffff888237c80000(0000) knlGS:0000000000000000
[17.666525] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[17.668664] CR2: 000055d4a39152e8 CR3: 000000010c596002 CR4: 0000000000770ee0
[17.671253] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[17.673634] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[17.676034] PKRU: 55555554
[17.677004] Call Trace:
[17.677877] add_all_parents+0x276/0x480
[17.679325] find_parent_nodes+0xfae/0x1590
[17.680771] btrfs_find_all_leafs+0x5e/0xa0
[17.682217] iterate_extent_inodes+0xce/0x260
[17.683809] ? btrfs_inode_flags_to_xflags+0x50/0x50
[17.685597] ? iterate_inodes_from_logical+0xa1/0xd0
[17.687404] iterate_inodes_from_logical+0xa1/0xd0
[17.689121] ? btrfs_inode_flags_to_xflags+0x50/0x50
[17.691010] btrfs_ioctl_logical_to_ino+0x131/0x190
[17.692946] btrfs_ioctl+0x104a/0x2f60
[17.694384] ? selinux_file_ioctl+0x182/0x220
[17.695995] ? __x64_sys_ioctl+0x84/0xc0
[17.697394] __x64_sys_ioctl+0x84/0xc0
[17.698697] do_syscall_64+0x33/0x40
[17.700017] entry_SYSCALL_64_after_hwframe+0x44/0xae
[17.701753] RIP: 0033:0x7f64e72761b7
[17.709355] RSP: 002b:00007ffefb067f58 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[17.712088] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f64e72761b7
[17.714667] RDX: 00007ffefb067fb0 RSI: 00000000c0389424 RDI: 0000000000000003
[17.717386] RBP: 00007ffefb06d188 R08: 000055d4a390d2b0 R09: 00007f64e7340a60
[17.719938] R10: 0000000000000231 R11: 0000000000000246 R12: 0000000000000001
[17.722383] R13: 0000000000000000 R14: 00000000c0389424 R15: 000055d4a38fd2a0
[17.724839] Modules linked in:

Fix the bug by detecting the inline extent item in add_all_parents and
skipping to the next extent item.