rbd: kill rbd_is_lock_supported()

Currently the exclusive lock is acquired only if the mapping is
writable, i.e. an image HEAD mapped in rw mode.  This means that we
don't acquire the lock for executing a read from a snapshot or an image
HEAD mapped in ro mode, even if lock_on_read is set.  This is somewhat
weird and inconsistent with "no automatic exclusive lock transfers"
mode, where the lock is acquired unconditionally.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>