LU-13699 mdt: Improve message reporting for mdt_identity.c

LU-13699 mdt: Improve message reporting for mdt_identity.c

This patch Improves Error handling and message reporting
for file lustre/mdt/mdt_identity.c

This patch also replaces ERR_PTR(PTR_ERR()) with ERR_CAST()
which is reported by coccinelle

Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: I59e262e743e7b1926fe75920239d6086c183b30f
Reviewed-on: https://review.whamcloud.com/39203
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>