os/FileStore: tolerate ENOTEMPTY on RMCOLL on non-btrfs

We may have a sequence like:

 RMCOLL foo
 MKCOLL foo
 create object in foo
 <crash>

On replay, we need to tolerate the ENOTEMPTY unless/until we have a way
to do a replay guard-ish thing on the collection creation/deletion
operations.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: improve debug output on snap collections

Signed-off-by: Sage Weil <sage@inktank.com>