fstests: fstest.c, fix compile warnings replace sprintf with snprintf

fstests: fstest.c, fix compile warnings replace sprintf with snprintf

Fixes the buffer overflow warnings, by using snprintf instead of
sprintf.

fstest.c:95:20: warning: '/file' directive writing 5 bytes into a region of size between 1 and 1024 [-Wformat-overflow=]
  sprintf(fname, "%s/file%d", dir, fnum);
                    ^~~~~
fstest.c:166:20: warning: '/file' directive writing 5 bytes into a region of size between 1 and 1024 [-Wformat-overflow=]
  sprintf(fname, "%s/file%d", dir, fnum);

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>