LU-13910 mdt: 0 for success in mdt_path_current()

LU-13910 mdt: 0 for success in mdt_path_current()

0 should be returned if no error is found in mdt_path_current(),
otherwise, the non-zero value will be treated as an error in
mdc_ioc_fid2path() and null will be returned by "lfs fid2path".

sanity.sh test_226c is added to verify this patch.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I1c10b023da9bbbb908dfb691fcea6e84ced67a8d
Reviewed-on: https://review.whamcloud.com/39688
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>