btrfs: stripe_alloc: do not read an inline backref off a keyed-ref extent item

btrfs: stripe_alloc: do not read an inline backref off a keyed-ref extent item

stripe_extents_owned_by() decides whether every extent in a stripe range
belongs to one inode by reading the single inline backref of each data
extent item.  A data extent's only backref is keyed rather than inline
when the leaf had no room to grow the item: insert_inline_extent_backref()
returns -EAGAIN and insert_extent_backref() adds a separate
EXTENT_DATA_REF item.  Such an extent item ends right after the header,
so the walk read the next item's bytes as an inline ref and
btrfs_get_extent_inline_ref_type() warned:

  BTRFS error (device dm-22): eb 81657856 iref 0x3a5c invalid extent inline ref type 1
  WARNING: CPU: 6 PID: 199813 at fs/btrfs/extent-tree.c:380 btrfs_get_extent_inline_ref_type+0xe0/0x190
   stripe_extents_owned_by+0x270/0x310
   btrfs_alloc_from_inode_stripe_run+0x48e/0x6a0
   find_free_extent+0xf9b/0x1870
   btrfs_reserve_extent+0x214/0x890
   __btrfs_prealloc_file_range+0xf1/0x470
   btrfs_fallocate+0xa99/0x10f0

The answer was already the conservative "not owned" (the garbage type is
not EXTENT_DATA_REF_KEY), so only the warning and the taint were wrong.
Check the item size before reading the inline ref and treat a keyed-ref
extent as not owned.

Assisted-by: Claude:claude-fable-5