LU-12137 osd-ldiskfs: use ERR_CAST in osd_oi_index_open()

LU-12137 osd-ldiskfs: use ERR_CAST in osd_oi_index_open()

In osd_oi_index_open() when dentry is invalid it is void casting
the invalid dentry being returned. This is what ERR_CAST was
invented for so use it.

Test-Parameters: trivial

Change-Id: I71016ff9c9dfc4408db8ab14576ba87dd6dc352d
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35455
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>