Check for bogus_page in vnode_pager_generic_getpages_done().

We now assert that a page is busy when updating its validity-tracking
state, but bogus_page is not busied during a getpages operation.

Reported by: syzkaller
Reviewed by: alc, kib
Discussed with: jeff
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22124