nfs: reject changes to resvport and sharecache during remount

Commit c8e47028 made it possible to change resvport/noresvport and
sharecache/nosharecache via a remount operation, neither of which should be
allowed.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Fixes: c8e47028 (nfs: Apply NFS_MOUNT_CMP_FLAGMASK to nfs_compare_remount_data)
Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>