The mp variable in xfs_file_compat_ioctl is only used when
BROKEN_X86_ALIGNMENT is define.  Remove it and just open code the
dereference in a few places.

Link: https://lore.kernel.org/r/20210203173009.462205-1-christian.brauner@ubuntu.com
Fixes: f736d93d ("xfs: support idmapped mounts")
Reviewed-by: NDarrick J. Wong <djwong@kernel.org>
Acked-by: NChristian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NChristian Brauner <christian.brauner@ubuntu.com>