xfs: remove the possibly unused mp variable in xfs_file_compat_ioctl

xfs: remove the possibly unused mp variable in xfs_file_compat_ioctl

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: f736d93d76d3 ("xfs: support idmapped mounts")
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>