mac80211_hwsim: Use kstrndup() in place of kasprintf()

syzbot reports a warning:

precision 33020 too large
WARNING: CPU: 0 PID: 9618 at lib/vsprintf.c:2471 set_precision+0x150/0x180 lib/vsprintf.c:2471
 vsnprintf+0xa7b/0x19a0 lib/vsprintf.c:2547
 kvasprintf+0xb2/0x170 lib/kasprintf.c:22
 kasprintf+0xbb/0xf0 lib/kasprintf.c:59
 hwsim_del_radio_nl+0x63a/0x7e0 drivers/net/wireless/mac80211_hwsim.c:3625
 genl_family_rcv_msg_doit net/netlink/genetlink.c:672 [inline]
 ...
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Thus it seems that kasprintf() with "%.*s" format can not be used for
duplicating a string with arbitrary length. Replace it with kstrndup().

Note that later this string is limited to NL80211_WIPHY_NAME_MAXLEN == 64,
but the code is simpler this way.

Reported-by:  <syzbot+6693adf1698864d21734@syzkaller.appspotmail.com>
Reported-by:  <syzbot+a4aee3f42d7584d76761@syzkaller.appspotmail.com>
Cc: stable@kernel.org
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Link: https://lore.kernel.org/r/20200410123257.14559-1-tuomas.tynkkynen@iki.fi


[johannes: add note about length limit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>