ioping: make block device support optional

This patch turns lack of block device size getter into warning. In this case
ioping woudnt handle block devices but still be able to do file requests.
Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>

ioping.c

@@ -612,7 +612,10 @@ off_t get_device_size(int fd, struct stat *st)
 	ret = ioctl(fd, DKIOCGMEDIAINFO, &dkmp);
 	blksize =  dkmp.dki_capacity * dkmp.dki_lbsize;
 #else
-# error no get disk size method
+# warning no get disk size method
+	ret = -1;
+	errno = ENOSYS;
+	blksize = 0;
 #endif
 	(void)fd;
 	(void)st;