9p/trans_fd: abort p9_read_work if req status changed

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>