xfs: rename variable mp to parsing_mp

Rename mp variable to parsisng_mp so it is easy to distinguish
between current mount point handle and handle for mount point
which mount options are being parsed.

Suggested-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Pavel Reichl <preichl@redhat.com>

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>