NFSD: Optimize nfsd4_encode_readv()

write_bytes_to_xdr_buf() is pretty expensive to use for inserting
an XDR data item that is always 1 XDR_UNIT at an address that is
always XDR word-aligned.

Since both the readv and splice read paths encode EOF and maxcount
values, move both to a common code path.

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