9p/trans_fd: abort p9_read_work if req status changed

[ Upstream commit e4ca13f7d075e551dc158df6af18fb412a1dba0a ]

p9_read_work would try to handle an errored req even if it got put to
error state by another thread between the lookup (that worked) and the
time it had been fully read.
The request itself is safe to use because we hold a ref to it from the
lookup (for m->rreq, so it was safe to read into the request data buffer
until this point), but the req_list has been deleted at the same time
status changed, and client_cb already has been called as well, so we
should not do either.

Link: http://lkml.kernel.org/r/1539057956-23741-1-git-send-email-asmadeus@codewreck.org


Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Reported-by:  <syzbot+2222c34dc40b515f30dc@syzkaller.appspotmail.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>