nvme: add set feature tracing support

A nvme connect command produces following trace.

Before:
/sys/kernel/debug/tracing# cat trace | grep feature
    kworker/5:1H-98      [005] ....  3221.294844: nvme_setup_cmd: nvme0: qid=0, cmdid=25, nsid=0, flags=0x0, 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/4:1H-124     [004] ....  3222.009186: nvme_setup_cmd: nvme0: qid=0, cmdid=17, nsid=0, flags=0x0, 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)

After:
/sys/kernel/debug/tracing# cat trace | grep feature
    kworker/0:1H-253     [000] ....   196.060509: nvme_setup_cmd: nvme0: qid=0, cmdid=29, nsid=0, flags=0x0, meta=0x0, cmd=(nvme_admin_set_features fid=0x7, sv=0x0, cdw11=0x70007)
    kworker/0:1H-253     [000] ....   196.763947: nvme_setup_cmd: nvme0: qid=0, cmdid=29, nsid=0, flags=0x0, 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>