sys/vfs/autofs: prevent assert on unmount.

sys/vfs/autofs: prevent assert on unmount.

Bring in below commit from NetBSD/autofs. This hasn't existed in
FreeBSD and DragonFly. This at least avoids potential panic via
assertion failure on unmount like PR 3062. Note that the assertion
in autofs_node_delete() is good. There might be mkdir related bug.

The GitHub link below may not be a valid URL in the future.

http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/fs/autofs/autofs_vfsops.c.diff?r1=1.2&r2=1.3
(https://github.com/NetBSD/src/commit/59fdd9b6d94c045e89f22f06c9a6f6d6c0fd60f2)