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