btrfs: add the missing error handling inside get_canonical_dev_path

In the Linux kernel, the following vulnerability has been resolved:

btrfs: add the missing error handling inside get_canonical_dev_path

Inside function get_canonical_dev_path(), we call d_path() to get the

final device path.

But d_path() can return error, and in that case the next strscpy() call

will trigger an invalid memory access.

Add back the missing error handling for d_path().