Btrfs: handle EAGAIN case properly in btrfs_drop_snapshot()

commit 90515e7f5d7d24cbb2a4038a3f1b5cfa2921aa17 upstream.

We may return early in btrfs_drop_snapshot(), we shouldn't
call btrfs_std_err() for this case, fix it.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>