fs: btrfs: Prevent error pointer dereference in list_subvolums()

If btrfs_read_fs_root() fails with -ENOENT, then we go to the next
entry.  Fine.  But if it fails for a different reason then we need
to clean up and return an error code.  In the current code it
doesn't clean up but instead dereferences "root" and crashes.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>