fuse: fix leaked notify reply

fuse_request_send_notify_reply() may fail if the connection was reset for
some reason (e.g. fs was unmounted).  Don't leak request reference in this
case.  Besides leaking memory, this resulted in fc->num_waiting not being
decremented and hence fuse_wait_aborted() left in a hanging and unkillable
state.

Fixes: 2d45ba38 ("fuse: add retrieve request")
Fixes: b8f95e5d

 ("fuse: umount should wait for all requests")
Reported-and-tested-by:  <syzbot+6339eda9cb4ebbc4c37b@syzkaller.appspotmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org> #v2.6.36