NFSD: Add nfsd4_encode_count4()

This is a synonym for nfsd4_encode_uint32_t() that matches the
name of the XDR type. It will get at least one more use in a
subsequent patch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>