pktgen: convert safe uses of strncpy() to strcpy() to avoid string truncation warning

GCC 8 complains:

net/core/pktgen.c: In function ‘pktgen_if_write’:
net/core/pktgen.c:1419:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->src_max, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1399:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->src_min, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1290:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->dst_max, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/pktgen.c:1268:4: warning: ‘strncpy’ output may be truncated copying between 0 and 31 bytes from a string of length 127 [-Wstringop-truncation]
    strncpy(pkt_dev->dst_min, buf, len);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is no bug here, but the code is not perfect either.  It copies
sizeof(pkt_dev->/member/) - 1 from user space into buf, and then does
a strcmp(pkt_dev->/member/, buf) hence assuming buf will be null-terminated
and shorter than pkt_dev->/member/ (pkt_dev->/member/ is never
explicitly null-terminated, and strncpy() doesn't have to null-terminate
so the assumption must be on buf).  The use of strncpy() without explicit
null-termination looks suspicious.  Convert to use straight strcpy().

strncpy() would also null-pad the output, but that's clearly unnecessary
since the author calls memset(pkt_dev->/member/, 0, sizeof(..)); prior
to strncpy(), anyway.

While at it format the code for "dst_min", "dst_max", "src_min" and
"src_max" in the same way by removing extra new lines in one case.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>