9p/trans_fd: put worker reqs on destroy

9p/trans_fd: put worker reqs on destroy

p9_read_work/p9_write_work might still hold references to a req after
having been cancelled; make sure we put any of these to avoid potential
request leak on disconnect.

Fixes: 10f61be1685d4 ("9p: Add refcount to p9_req_t")
Link: http://lkml.kernel.org/r/1539057956-23741-2-git-send-email-asmadeus@codewreck.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Reviewed-by: Tomas Bortoli <tomasbortoli@gmail.com>