mds: fix null_snapflush with multiple intervening snaps

mds: fix null_snapflush with multiple intervening snaps

The client is allowed to not send a snapflush if there is no dirty metadata
to write for a given snap.  However, the mds can only look up inodes by
the last snapid in the interval.  So, when doing a null_snapflush (filling
in for snapflushes the client didn't send), we have to walk forward through
intervening snaps until we find the right inode.

Note that this means we will call _do_snap_update multiple times on the
same inode, but with different snapids.

Add unit test to check this.

Signed-off-by: Sage Weil <sage@newdream.net>