btrfs-progs: btrfs_close_devices: only fsync if device->writeable is set

Prevent unnecessary error from failing fsync(), if opened read only.

Performed 'grep "writeable = " *.h *.c' to make sure there were no odd
situations where fsync() might still be desired here.  They're all straight-
forward.  The only situation where writeable will be 0 is if btrfs_open_devices
is given flags without O_RDWR.  There is no situation where a writeable volume
temporarily becomes unwriteable, or anything like that.  Given that it's being
opened O_RDWR, there's no reason to attempt fsync().

utils.c

   int btrfs_add_to_fsid() {
      ...
      device->writeable = 1;

volumes.c

   int btrfs_close_devices() {
      ...
      while (!list_empty(&fs_devices->devices)) {
         ...
         // just after the fsync() being patched
267:     device->writeable = 0;
   ...
   int btrfs_open_devices() {
      ...
      list_for_each_entry(device, &fs_devices->devices, dev_list) {
         ...
         if (flags & O_RDWR)
332:        device->writeable = 1

kernel btrfs_close_devices() does not have a corresponding fsync() that I see.

Signed-off-by: James Harvey <jamespharvey20@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

volumes.c

@@ -254,7 +254,7 @@ again:
 		device = list_entry(fs_devices->devices.next,
 				    struct btrfs_device, dev_list);
 		if (device->fd != -1) {
-			if (fsync(device->fd) == -1) {
+			if (device->writeable && fsync(device->fd) == -1) {
 				warning("fsync on device %llu failed: %m",
 					device->devid);
 				ret = -errno;