net: buf: linearize: Never return -ENOMEM, just do what user asked to

Don't try to find "errors" in the values of dst_len and len params
passed to net_buf_linearize(). Instead, do what entails with the
common sense from the values passed in, specifically:

1. Never read more than dst_len (or it would lead to buffer
overflow).
2. It's absolutely ok to read than specified by "len" param, that's
why this function returns number of bytes read in the first place.

The motivation for this change is that it's not useful with its
current behavior. For example, a number of Ethernet drivers linearize
a packet to send, but each does it with its own duplicated adhoc
routine, because net_buf_linearize() would just return error for the
natural use of:

net_buf_linearize(buf, sizeof(buf), pkt->frags, 0, sizeof(buf));

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>