mds: fix client cap condition

mds: fix client cap condition

In 551a12f52e36 we fixed a bug with cow_inode() where the
cap->client_follows didn't match last precisely.  Instead, we compare
to first.  But the == is too strict.. cap follows that is equal _or_older_
than the clone's first should be copied to the clone inode.

This fixes the simple test case
 $ echo asdf > bar ; mkdir .snap/bar ; rm bar ; cat .snap/bar/bar
 asdf
(Previously we would get nothing unless we waited for the cap to flush on
its own.)