librbd: remove image header lock assertions

librbd: remove image header lock assertions

This assertions can sporadically fail if the watch is lost and
recovered in the background. Upon a true loss of the lock, the
client would either be blacklisted or it would have completed
all in-flight ops before releasing.

Fixes: http://tracker.ceph.com/issues/18244
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit ce4f2a52ec0a794d89e7576b59c9b9aefe3db288)

Conflicts:
	src/librbd/operation/SnapshotCreateRequest.cc: rbd class
	   does not support the snapshot namespaces in Jewel, skip
	   the corresponding argument