NFSD: Adjust filename length argument of nfsd_lookup

Clean up: adjust the sign of the length argument of nfsd_lookup and
nfsd_lookup_dentry, for consistency with recent changes.  NFSD version
4 callers already pass an unsigned file name length.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-By: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>