Generalize vn_get_ino() to allow filesystems to use custom vnode
producer, instead of hard-coding VFS_VGET().  New function, which
takes callback, is called vn_get_ino_gen(), standard callback for
vn_get_ino() is provided.

Convert inline copies of vn_get_ino() in msdosfs and cd9660 into the
uses of vn_get_ino_gen().

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
In kern_linkat(), avoid passing doomed vnode to the VOP.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks