9p-trans_fd: don't do fs segment mangling in p9_fd_poll()

p9_fd_poll() is never called with user pointers and f_op->poll()
doesn't expect its arguments to be from userland.  There's no need to
set kernel ds before calling f_op->poll() from p9_fd_poll().  Remove
it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>