dump_write() returns ENXIO if the dump is trying to be written outside
of the device boundry.
While this is generally ok, the problem is that all the consumers
handle similar cases (and expect to catch) ENOSPC for this (for a
reference look at minidumpsys() and dumpsys() constructions). That
ends up in consumers not recognizing the issue and amd64 failing to
retry if the number of pages grows up during minidump.
Fix this by returning ENOSPC in dump_write() and while here add some
more diagnostic on involved values.

Sponsored by: Sandvine Incorporated
In collabouration with: emaste
Approved by: re (kib)
MFC after: 10 days