blktests: Fix compilation warning

blktests: Fix compilation warning

Fix strncpy length to "sizeof(buf) - 1" to avoid the compiler warning:
cc  -O2 -Wall -Wshadow  -o sg/syzkaller1 sg/syzkaller1.c
sg/syzkaller1.c: In function ‘syz_open_dev.constprop’:
sg/syzkaller1.c:204:16: warning: ‘strncpy’ specified bound 1024 equals
destination size [-Wstringop-truncation]
     NONFAILING(strncpy(buf, (char*)a0, sizeof(buf)));
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sg/syzkaller1.c:143:7: note: in definition of macro ‘NONFAILING’
       __VA_ARGS__;                                                     \
       ^~~~~~~~~~~

Since the last character of buf is forced set to 0, there is no
functional change introduced by this patch.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>