btrfs: sync ioctl, handle errors after transaction start

The version merged to 3.19 did not handle errors from start_trancaction
and could pass an invalid pointer to commit_transaction.

Fixes: 6b5fe46d ("btrfs: do commit in sync_fs if there are pending changes")
Reported-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>