mds: protect open file table against partial omap update

mds: protect open file table against partial omap update

When omap update is too large for single osd request, mds first stores
the update in special journal keys in the omap, then sends requests to
update the original omap. The first request of the second phase also
updates a flag in omap header, indicating that update is journaled, The
last request of the second phase removes journal keys and removes the
journal flag in omap header

Signed-off-by: Yan, Zheng <zyan@redhat.com>