LU-9859 llite: simplify pcc_fname_list_parse()

LU-9859 llite: simplify pcc_fname_list_parse()

Now that pcc_fname_list_parse() is passed a mutable string, mutable
parsing with standard tools can be used.

Test-Parameters: trivial testlist=sanity-pcc
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I8da6720ae6822c410a339b5fff9fdd4256bb3685
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50840
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>