os/ObjectStore: fix RMATTRS encoding

Apparently nobody uses this!

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: fix getattr return value when using omap

The return value should be the length of the value, even when it was
stored in omap.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/osd_types: init SnapSet::seq in ctor

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-osd-fixes' into next

Reviewed-by: Samuel Just <sam.just@inktank.com>