FileStore::_setattrs no need to grab an Index lock for the omap operations

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore::_setattrs: only do omap operations if necessary

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: add _fsetattrs

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore::_setattrs: use _fsetattrs

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore::_clone: use _fsetattrs rather than _setattrs

The omap portion of the clone happened above in DBObjectMap::clone.
Only the fs stored attrs need to be explicitely copied.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branch 'upstream/wip_clone_attrs'

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