vm: Fix some bugs in the page busying code

In vm_page_busy_acquire(), load the object pointer using
atomic_load_ptr() as we do elsewhere. Per the comment, the object
identity must be consistent across sleeps.

In vm_page_grab_sleep(), pass the correct pindex to
_vm_page_busy_sleep(). The pindex is used to re-check the page's
identity before going to sleep. In particular, vm_page_grab_sleep() is
used in unlocked grab, so the object lock is not necessarily held when
verifying the page's identity, and the pindex may change if the page is
moved, or freed and re-allocated. I believe this can result in spurious
VM_PAGER_FAILs from vm_page_grab_valid_unlocked() or early termination
of vm_page_grab_pages_unlocked().

In vm_page_grab_pages(), pass the correct pindex to
vm_page_grab_sleep(). Otherwise I believe vm_page_grab_pages() will
effectively spin when attempting to busy a busy page after the first
index in the range.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27607