bluestore: the exhausted check in BitMapZone can be lock-less.

bluestore: the exhausted check in BitMapZone can be lock-less.

Before the patch BitMapZone::is_exhausted() required from its
callers to acquire appropriate lock. However, fulfilling this
condition is not really necessary to use the method correctly
while it can significantly hurt performance.

The change allows BitMapAreaLeaf::child_check_n_lock() to not
acquire the lock while examining zones for being exhausted.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>