LU-9859 lnet: simplify cfs_expr_list_parse()

LU-9859 lnet: simplify cfs_expr_list_parse()

If we dup the string first, we can use a mutating approach to parsing,
allowing us to use standard tools like strsep, strcmp, kstrtouint,
etc.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I40a1f7bc65bd122d22cd53cf3c645f4a3730f82e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50843
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>