Only assert the length of the passed bio in the mdstart_vnode() when
the bio is unmapped, so we must map the bio pages into pbuf.  This
works around the geom classes which do not follow the MAXPHYS limit on
the i/o size, since such classes do not know about unmapped bios
either.

Reported by:	Paolo Pinto <paolo.pinto@netasq.com>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Return the correct IEEE 1275 code for "nextprop".
Fix detection of EOF in kern_physio().  If bio_length was clipped by
the excess code in g_io_check(), bio_resid is also truncated by
g_io_deliver().  As result, bufdonebio() assigns truncated value to
the buffer b_resid field.

Use the residual bio_completed to calculate buffer b_resid from
b_bcount in bufdonebio(), instead of bio_resid, calculated from
bio_length in g_io_deliver().

The issue is seemingly caused by the code rearrange into g_io_check(),
which is not present in stable/10.  The change still looks as the
useful change to have in 10 nevertheless.

Reported by:	Stefan Hegnauer <stefan.hegnauer@gmx.ch>
Tested by:	pho, Stefan Hegnauer <stefan.hegnauer@gmx.ch>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week