ANDROID: mm: assert that mmap_lock is taken exclusively in vm_write_begin

vm_write_{begin|end} has to be called when mmap_lock is taken
exlusively. Add an assert statement in vm_write_begin to enforce
that. free_pgtables can free page tables without exclusive mmap_lock
if the vma was isolated, therefore avoid assertions in such cases.

Bug: 257443051
Change-Id: Ie81aefe025c743cda6f66717d2f08f4d78a55608
Signed-off-by: Suren Baghdasaryan <surenb@google.com>