msg/simple/Pipe: avoid returning 0 on poll timeout

msg/simple/Pipe: avoid returning 0 on poll timeout

If poll times out it will return 0 (no data to read on socket).  In
165e5abdbf6311974d4001e43982b83d06f9e0cc we changed tcp_read_wait from
returning -1 to returning -errno, which means we return 0 instead of -1
in this case.

This makes tcp_read() get into an infinite loop by repeatedly trying to
read from the socket and getting EAGAIN.

Fix by explicitly checking for a 0 return from poll(2) and returning
EAGAIN in that case.

Fixes: http://tracker.ceph.com/issues/18184
Signed-off-by: Sage Weil <sage@redhat.com>