nfsd4: fix crash on unknown operation number

commit 51904b08072a8bf2b9ed74d1bd7a5300a614471d upstream.

Unknown operation numbers are caught in nfsd4_decode_compound() which
sets op->opnum to OP_ILLEGAL and op->status to nfserr_op_illegal.  The
error causes the main loop in nfsd4_proc_compound() to skip most
processing.  But nfsd4_proc_compound also peeks ahead at the next
operation in one case and doesn't take similar precautions there.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>