nfsd: Correct a comment for NFSD_MAY_ defines location

Those are now defined in fs/nfsd/vfs.h

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>