ANDROID: mm: preserve vma->anon_vma after MREMAP_DONTUNMAP for SPF

The optimizations [1] and [2] to reset vma->anon_vma during
MREMAP_DONTUNMAP can affect speculative page fault handler. If
vma->anon_vma reset happens after do_anonymous_page verified no
changes to the vma and obtained the ptl lock but before it calls
page_add_new_anon_rmap() then __page_set_anon_rmap() will stumble
on BUG_ON(!anon_vma). Disable these optimizations if SPF is enabled
to avoid such situations. As a result the reverse map walk will
consider the old VMA as it did before these optimizations were
introduced.

[1] 1583aa27 ("mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success")
[2] ee8ab190 ("mm: rmap: explicitly reset vma->anon_vma in unlink_anon_vmas()")

Bug: 257443051
Change-Id: I4e7611137f4a49c94bfe73532b4b06cbb0d2405b
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Lee Jones <joneslee@google.com>