SUNRPC: Clean up helpers xdr_set_iov() and xdr_set_page_base()

Allow xdr_set_iov() to set a base so that we can use it to set the
cursor to a specific position in the kvec buffer.

If the new base overflows the kvec/pages buffer in either xdr_set_iov()
or xdr_set_page_base(), then truncate it so that we point to the end of
the buffer.

Finally, change both function to return the number of bytes remaining to
read in their buffers.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>