ANDROID: mm: skip pte_alloc during speculative page fault

Speculative page fault checks pmd to be valid before starting to handle
the page fault and pte_alloc() should do nothing if pmd stays valid.
If pmd gets changed during speculative page fault, we will detect the
change later and retry with mmap_lock. Therefore pte_alloc() can be
safely skipped and this prevents the racy pmd_lock() call which can
access pmd->ptl after pmd was cleared.

Bug: 257443051
Change-Id: Iec57df5530dba6e0e0bdf9f7500f910851c3d3fd
Signed-off-by: Suren Baghdasaryan <surenb@google.com>