mds: Revert "properly setup client_need_snapflush for snap inode"

mds: Revert "properly setup client_need_snapflush for snap inode"

This reverts commit 9fb61cd65895dbdb92cb46d24be8bf79ce57409d. That
commit handle a case that cap message flushes dirty caps and releases
the same same caps. MDCache::cow_inode() loses track of the released
caps and does not create snap inode.

Previous commit make mds track dirty snap caps as whole. Above case
become "cap message releases all write caps". If client can release
all wirte caps, it should have already sent all snap flush message.

 Conflicts:
	src/mds/Locker.cc

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