fs/super.c: improve get_tree() error message

seeing an odd bug where we fail to correctly return an error from
.get_tree():

https://syzkaller.appspot.com/bug?extid=c0360e8367d6d8d04a66



we need to be able to distinguish between accidently returning a
positive error (as implied by the log) and no error.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>