nvmet: use min of device_path and disk len

In function __assign_req_name() instead of using the DEVICE_NAME_LEN in
strncpy() use min of DISK_NAME_LEN and strlen(req->ns->device_path).

This is needed to turn off the following warnings:-

In file included from drivers/nvme/target/core.c:14:
In function ‘__assign_req_name’,
    inlined from ‘trace_event_raw_event_nvmet_req_init’ at drivers/nvme/target/./trace.h:58:1:
drivers/nvme/target/trace.h:52:3: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
   strncpy(name, req->ns->device_path, DISK_NAME_LEN);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘__assign_req_name’,
    inlined from ‘perf_trace_nvmet_req_complete’ at drivers/nvme/target/./trace.h:100:1:
drivers/nvme/target/trace.h:52:3: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
   strncpy(name, req->ns->device_path, DISK_NAME_LEN);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘__assign_req_name’,
    inlined from ‘perf_trace_nvmet_req_init’ at drivers/nvme/target/./trace.h:58:1:
drivers/nvme/target/trace.h:52:3: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
   strncpy(name, req->ns->device_path, DISK_NAME_LEN);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘__assign_req_name’,
    inlined from ‘trace_event_raw_event_nvmet_req_complete’ at drivers/nvme/target/./trace.h:100:1:
drivers/nvme/target/trace.h:52:3: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
   strncpy(name, req->ns->device_path, DISK_NAME_LEN);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>