nvmet: add set feature tracing support

A nvme connect command produces following trace from the target side.

Before:
    kworker/0:1H-56      [000] ....  9012.155139: nvmet_req_init: nvmet1: qid=0, cmdid=16, nsid=0, flags=0x40, meta=0x0, cmd=(nvme_admin_set_features, cdw10=07 00 00 00 07 00 07 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00)
    kworker/0:1H-56      [000] ....  9012.872272: nvmet_req_init: nvmet1: qid=0, cmdid=13, nsid=0, flags=0x40, meta=0x0, cmd=(nvme_admin_set_features, cdw10=0b 00 00 00 00 09 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00)

cmdline:/sys/kernel/debug/tracing# cat trace | grep feature
    kworker/0:1H-56      [000] ....   203.493914: nvmet_req_init: nvmet1: qid=0, cmdid=29, nsid=0, flags=0x40, meta=0x0, cmd=(nvme_admin_set_features, fid=0x7, sv=0x0, cdw11=0x70007)
    kworker/0:1H-56      [000] ....   204.197079: nvmet_req_init: nvmet1: qid=0, cmdid=29, nsid=0, flags=0x40, meta=0x0, cmd=(nvme_admin_set_features, fid=0xb, sv=0x0, cdw11=0x900)

Using ',' to separate different field like others in
nvmet_trace_admin_get_features.

Signed-off-by: Hou Pu <houpu.main@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>