autofs: fix the return value of autofs4_fill_super

While kzallocing sbi/ino fails, it should return -ENOMEM.

And it should return the err value from autofs_prepare_pipe.

Signed-off-by: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>