file: poke ustream after starting deferred program

file: poke ustream after starting deferred program

When we're starting a deferred request, the related input ustream might
have gone into read_blocked mode because incoming client request data
exhausted the ustreams internal buffer space. When this happens, edge
triggered uloop read events are "lost" and never re-triggered causing
the script input to never complete.

In order to avoid that deadlock situation, manually poke the input
ustream using ustream_poll() after invoking client_poll_post_data()
which should have drained (some) of the buffered input ustream contents.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 2ee323c01079248baa9465969df9e25b5fb68cdf)