nfsd: fix oops on unsupported operation

I'm hitting the BUG in nfsd4_max_reply() at fs/nfsd/nfs4proc.c:2495 when
client sends an operation the server doesn't support.

in nfsd4_max_reply() it checks for NULL rsize_bop but a non-supported
operation wouldn't have that set.

Cc: Kinglong Mee <kinglongmee@gmail.com>
Fixes: 2282cd2c

 "NFSD: Get response size before operation..."
Signed-off-by: J. Bruce Fields <bfields@redhat.com>