btrfs-progs: add fd version of device_get_partition_size

The helper wraps a raw ioctl but some users may already have the fd and
not necessarily the path. Add a suitable helper for convenience.

Signed-off-by: David Sterba <dsterba@suse.com>

common/device-utils.c

@@ -289,6 +289,16 @@ u64 btrfs_device_size(int fd, struct stat *st)
 /*
  * Read partition size using the low-level ioctl
  */
+u64 device_get_partition_size_fd(int fd)
+{
+	u64 result;
+
+	if (ioctl(fd, BLKGETSIZE64, &result) < 0)
+		return 0;
+
+	return result;
+}
+
 u64 device_get_partition_size(const char *dev)
 {
 	u64 result;

common/device-utils.h

@@ -34,6 +34,7 @@
 int device_discard_blocks(int fd, u64 start, u64 len);
 int device_zero_blocks(int fd, off_t start, size_t len);
 u64 device_get_partition_size(const char *dev);
+u64 device_get_partition_size_fd(int fd);
 int device_get_queue_param(const char *file, const char *param, char *buf, size_t len);
 
 /*