osd: Handle omap and data digests independently

osd: Handle omap and data digests independently

Caused by: be078c8b7b131764caa28bc44452b8c5c2339623
The original attempt above to fix the omap_digest handling when
data_digest isn't present had 2 errors.  First, it checked
is_data_digest() and is_omap_digest() instead of digest_present and
omap_digest_present which indicate the source digest is available.
Second, MAYBE could only be set if both digests are available.

Fixes: http://tracker.ceph.com/issues/24366
Signed-off-by: David Zafman <dzafman@redhat.com>