librbd: use snap_lock to protect ImageCtx->flags

librbd: use snap_lock to protect ImageCtx->flags

This is another step towards eliminating md_lock from the writeback
path. Almost all the places that use ImageCtx->flags already use
snap_lock, so there's no need to create a new lock. For the others,
add a helper, test_flags() that acquires the lock, similar to
test_features().

This also makes sure we look up flags of the snapshot we're operating
on, instead of those for head.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>