NFSD: Use xdr_inline_decode() to decode NFSv3 symlinks

Replace the check for buffer over/underflow with a helper that is
commonly used for this purpose. The helper also sets xdr->nwords
correctly after successfully linearizing the symlink argument into
the stream's scratch buffer.

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