LU-3411 osd: fsfilt_get_ops returns an error, not NULL

osd_mount() should be able to recognize this properly.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Ife1f52cf264fd6041dfc01606c6e48e47f2f8ff0
Reviewed-on: http://review.whamcloud.com/6496
Tested-by: Hudson
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>