osd: allow greedy get_write() for ObjectContext locks

There are several lockers that need to take a write lock
because there is an operation that is already in progress and
know it is safe to do so.  In particular, they need to skip
the starvation checks (op waiters, backfill waiting).

Signed-off-by: Sage Weil <sage@redhat.com>
osd/ReplicatedPG: greedily take write_lock for copyfrom finish, snapdir

In the cases where we are taking a write lock and are careful
enough that we know we should succeed (i.e, we assert(got)),
use the get_write_greedy() variant that skips the checks for
waiters (be they ops or backfill) that are normally necessary
to avoid starvation.  We don't care about staration here
because our op is already in-progress and can't easily be
aborted, and new ops won't start because they do make those
checks.

Fixes: #8889
Signed-off-by: Sage Weil <sage@redhat.com>
osd/ReplicatedPG: debug obc locks

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2140 from ceph/wip-8889

osd: greedily get obc write lock in some cases

Reviewed-by: Greg Farnum <greg@inktank.com>