nfs_common: rename nfslocalio nfs_uuid_lock to nfs_uuids_lock

This global spinlock protects all nfs_uuid_t relative to the global
nfs_uuids list.  A later commit will split this global spinlock so
prepare by renaming this lock to reflect its intended narrow scope.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>