xfs: fix type confusion in xfs_ioc_swapext

xfs: fix type confusion in xfs_ioc_swapext

Without this check, the following XFS_I invocations would return bad
pointers when used on non-XFS inodes (perhaps pointers into preceding
allocator chunks).

This could be used by an attacker to trick xfs_swap_extents into
performing locking operations on attacker-chosen structures in kernel
memory, potentially leading to code execution in the kernel.  (I have
not investigated how likely this is to be usable for an attack in
practice.)

Signed-off-by: Jann Horn <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Chinner <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>