LU-9325 fld: replace simple_strto* with kstr* functions

LU-9325 fld: replace simple_strto* with kstr* functions

The fldb debugfs files use simple_strto* to parse input from the
user. simple_strto* is considered obsolete so replace it with
the equivalent kstrto* functions.

Change-Id: I6d32939152ee0d65df4ec45937d7d0be03b8274e
Test-Parameters: trivial env=ONLY=68 testlist=conf-sanity
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/39498
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>