mds: properly setup client_need_snapflush for snap inode

mds: properly setup client_need_snapflush for snap inode

MDCache::cow_inode() checks "cap->issued() & CEPH_CAP_ANY_WR" to decide
if it needs to setup client_need_snapflush for the new snap inode. If
cap message flushes dirty caps and releases the same caps, cap->issued()
may have no WR caps when MDCache::cow_inode() gets called. The solution
is temporarily setting NEEDSNAPFLUSH on Capability::state.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit 9fb61cd65895dbdb92cb46d24be8bf79ce57409d)