blockdev-util: add simple wrapper around BLKSSZGET

Just adds some typesafety and generates an error if the field is not
initialized in the block device yet.

src/shared/blockdev-util.c

@@ -762,3 +762,18 @@ int blockdev_reread_partition_table(sd_device *dev) {
 
         return 0;
 }
+
+int blockdev_get_sector_size(int fd, uint32_t *ret) {
+        int ssz = 0;
+
+        assert(fd >= 0);
+        assert(ret);
+
+        if (ioctl(fd, BLKSSZGET, &ssz) < 0)
+                return -errno;
+        if (ssz <= 0) /* make sure the field is initialized */
+                return log_debug_errno(SYNTHETIC_ERRNO(EIO), "Block device reported invalid sector size %i.", ssz);
+
+        *ret = ssz;
+        return 0;
+}