nfsd: enforce upper limit for namelen in __cld_pipe_inprogress_downcall()

This patch is intended to go on top of "nfsd: return -EINVAL when
namelen is 0" from Li Lingfeng.  Li's patch checks for 0, but we should
be enforcing an upper bound as well.

Note that if nfsdcld somehow gets an id > NFS4_OPAQUE_LIMIT in its
database, it'll truncate it to NFS4_OPAQUE_LIMIT when it does the
downcall anyway.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>