VM area marked with VM_WRITE means that pages may be written, but mmap page write may ne ver happen.
It should delay layout write until the actual modification on the file happen in ->page_mkwrite().
Otherwise, it will trigger panic for PCC-RO sanity-pcc test_21f().

Fixes: f2d1c4ee4 ("LU-14647 flr: mmap write/punch does not stale other mirrors")