btrfs: set page extent mapped after read_folio in relocate_one_page

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

btrfs: set page extent mapped after read_folio in relocate_one_page

One of the CI runs triggered the following panic

assertion failed: PagePrivate(page) && page->private, in fs/btrfs/subpage.c:229
------------[ cut here ]------------
kernel BUG at fs/btrfs/subpage.c:229!
Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
CPU: 0 PID: 923660 Comm: btrfs Not tainted 6.5.0-rc3+ #1
pstate: 61400005 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
pc : btrfs_subpage_assert+0xbc/0xf0
lr : btrfs_subpage_assert+0xbc/0xf0
sp : ffff800093213720
x29: ffff800093213720 x28: ffff8000932138b4 x27: 000000000c280000
x26: 00000001b5d00000 x25: 000000000c281000 x24: 000000000c281fff
x23: 0000000000001000 x22: 0000000000000000 x21: ffffff42b95bf880
x20: ffff42b9528e0000 x19: 0000000000001000 x18: ffffffffffffffff
x17: 667274622f736620 x16: 6e69202c65746176 x15: 0000000000000028
x14: 0000000000000003 x13: 00000000002672d7 x12: 0000000000000000
x11: ffffcd3f0ccd9204 x10: ffffcd3f0554ae50 x9 : ffffcd3f0379528c
x8 : ffff800093213428 x7 : 0000000000000000 x6 : ffffcd3f091771e8
x5 : ffff42b97f333948 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : ffff42b9556cde80 x0 : 000000000000004f
Call trace:
btrfs_subpage_assert+0xbc/0xf0
btrfs_subpage_set_dirty+0x38/0xa0
btrfs_page_set_dirty+0x58/0x88
relocate_one_page+0x204/0x5f0
relocate_file_extent_cluster+0x11c/0x180
relocate_data_extent+0xd0/0xf8
relocate_block_group+0x3d0/0x4e8
btrfs_relocate_block_group+0x2d8/0x490
btrfs_relocate_chunk+0x54/0x1a8
btrfs_balance+0x7f4/0x1150
btrfs_ioctl+0x10f0/0x20b8
__arm64_sys_ioctl+0x120/0x11d8
invoke_syscall.constprop.0+0x80/0xd8
do_el0_svc+0x6c/0x158
el0_svc+0x50/0x1b0
el0t_64_sync_handler+0x120/0x130
el0t_64_sync+0x194/0x198
Code: 91098021 b0007fa0 91346000 97e9c6d2 (d4210000)

This is the same problem outlined in 17b17fcd6d44 ("btrfs:
set_page_extent_mapped after read_folio in btrfs_cont_expand") , and the
fix is the same. I originally looked for the same pattern elsewhere in
our code, but mistakenly skipped over this code because I saw the page
cache readahead before we set_page_extent_mapped, not realizing that
this was only in the !page case, that we can still end up with a
!uptodate page and then do the btrfs_read_folio further down.

The fix here is the same as the above mentioned patch, move the
set_page_extent_mapped call to after the btrfs_read_folio() block to
make sure that we have the subpage blocksize stuff setup properly before
using the page.