This reverts commit d91f2438.

The intent of issue_seq is to distinguish between mds->client messages that
(re)create the cap and those that do not, which means we should _only_ be
updating that value in the create paths.  By updating it in handle_cap_grant,
we reset it to zero, which then breaks release.

The larger question is what workload/problem made me think it should be
updated here...
Signed-off-by: Sage Weil <sage@newdream.net>