g_access: deal with races created by geoms that drop the topology lock

The problem is that g_access() must be called with the GEOM topology
lock held. And that gives a false impression that the lock is indeed
held across the call. But this isn't always true because many classes,
ZVOL being one of the many, need to drop the lock. It's either to
perform an I/O on the first open or to acquire a different lock (like in
g_mirror_access).

That, of course, can break many assumptions. For example,
g_slice_access() adds an extra exclusive count on the first open. As
described above, an underlying geom may drop the topology lock and that
would open a race with another thread that would also request another
extra exclusive count. In general, two consumers may be granted
incompatible accesses.

To avoid this problem the code is changed to mark a geom with special
flag before calling its access method and clear the flag afterwards. If
another thread sees that flag, then it means that the topology lock has
been dropped (either by the geom in question or downstream from it), so
it is not safe to make another access call. So, the second thread would
use g_topology_sleep() to wait until the flag is cleared and only then
would it proceed with the access.

Also see http://docs.freebsd.org/cgi/mid.cgi?809d9254-ee56-59d8-69a4-08838e985cea

PR: 225960
Reported by: asomers
Reviewed by: markj, mav
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D14533