LU-9325 obd: replace simple_strtoul()

LU-9325 obd: replace simple_strtoul()

Replace the use of simple_strtoul() in filename_is_volatile() with
sscanf(). This change also strengthens the checking of the format
of the volatile file's name.

Remove the use of simple_strtoul() from target_name2index() as well.

Change-Id: I4bf15af00d6cecdd46eba3cca5df0fbeb473d9ce
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49910
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>