btrfs: free path at an earlier point in btrfs_get_extent

trace_btrfs_get_extent() has nothing to do with path, place
btrfs_free_path ahead so that we can unlock path on error.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>