LU-9859 ptlrpc: simplify nrs_tbf_opcode_list_parse()

LU-9859 ptlrpc: simplify nrs_tbf_opcode_list_parse()

If nrs_tbf_opcode_list_parse() duplicates the string it is passed, it
can use standard mutating functions for parsing the string.

Test-Parameters: trivial
Test-Parameters: testlist=sanityn
Test-Parameters: testlist=sanity
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4acb57de52abde97fa9c2d133cf10a432b12f604
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50837
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>