client: Don't signal requests already handled

The assertion failure reported in #4530 is triggered
by the following:

1. client sends request
2. mds sends unsafe reply
3. before request gets journaled, mds is killed
4. mds restarts
5. client reconnect triggers session close
6. session close does kick_requests()
7. kick_requests tries to signal caller that doesn't exist.

This fix avoids signaling a caller if the unsafe reply
has been received and the make_request() function has completed.
We do this by setting the caller_cond to null once the caller
is woken up, and only signal the caller in kick_requests if
caller_cond is non-null.  This avoids trying to resend requests
listed in mds_request but that have already received unsafe replies.
The unsafe requests are handled by resend_unsafe_requests() code,
so skipping those requests is allowable.

Fixes #4530.
Signed-off-by: Sam Lang <sam.lang@inktank.com>