LinuxKPI: fix sg_pcopy_from_buffer()

[bz]

Summary

In sg_pcopy_from_buffer() is an error in that skip can underflow
and lead to bogus page arithmetics which may lead to memory corruption
or more likely panics. Once we found a s/g page to copy into there
is nothing to skip anymore so simply set skip to 0.

Sponsored by: The FreeBSD Foundation
MFC after: 5 days

[• hselasky]

Don't forget to MFC this change.