librbd: avoid failing IO with -ESHUTDOWN when disabling exclusive-lock

librbd: avoid failing IO with -ESHUTDOWN when disabling exclusive-lock

When dynamically disabling the exclusive-lock feature with in-flight IO,
it was previously possible for IO to fail with -ESHUTDOWN when it
attempts to acquire the lock due to the PreReleaseRequest setting the
lock required flag. There is also the possibility for a race with the
first IO racing with exclusive-lock shutdown when the lock was not
already acquired.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>