nfsd : Define NFSD only when FILE_LOCKING is enabled

Enable NFSD only when FILE_LOCKING is enabled, since we don't want to
support NFSD without FILE_LOCKING.

Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>