msg/Pipe: goto fail_unlocked on early failures in accept()

Instead of duplicating an incomplete cleanup sequence (that does not
clear_pipe()), goto fail_unlocked and do the cleanup in a generic way.
s/rc/r/ while we are here.

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: reaper: make sure pipe has been cleared (under pipe_lock)

All paths to pipe shutdown should have cleared the con->pipe reference
already.  Assert as much.

Also, do it under pipe_lock!

Signed-off-by: Sage Weil <sage@inktank.com>