init: fix error check in clean_path()

init_stat() returns 0 on success, same as vfs_lstat().  When it replaced
vfs_lstat(), the '!' was dropped.

Fixes: 716308a5

 ("init: add an init_stat helper")
Signed-off-by: Barret Rhoden <brho@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>