ANDROID: mm: remove sequence counting when mmap_lock is not exclusively owned

In a number of cases vm_write_{begin|end} is called while mmap_lock is
not owned exclusively. This is unnecessary and can affect correctness of
the sequence counting protecting speculative page fault handlers. Remove
extra calls.

Bug: 257443051
Change-Id: I1278638a0794448e22fbdab5601212b3b2eaebdc
Signed-off-by: Suren Baghdasaryan <surenb@google.com>