LU-981 llite: fix loop back device writeback

LU-981 llite: fix loop back device writeback

In vvp_mmap_locks(), we should distinguish if the write is from
loopback device, and to not try to find mmap region in this case.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I4409e460b77576e07e279200ccc69f62cc48ad0f
Reviewed-on: http://review.whamcloud.com/2010
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>