LU-14709 pcc: VM_WRITE should not trigger layout write

LU-14709 pcc: VM_WRITE should not trigger layout write

VM area marked with VM_WRITE means that pages may be written, but
mmap page write may never 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")
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I1cbfef8a4ed7e2c718324fd8a21bafd6157b5f0c
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44483
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>