msg/simple/Pipe: avoid infinite loop in Pipe::do_recv()

msg/simple/Pipe: avoid infinite loop in Pipe::do_recv()

I found that, sometimes an OSD thread uses 100% CPU after cutting network
between OSD and client. recv(2) in Pipe::do_recv() keeps returning -EAGAIN,
which causes infinite loop. the call trace is:

 Pipe::do_recv (...)
 Pipe::buffered_recv (...)
 Pipe::tcp_read_nonblocking (...)
 Pipe::tcp_read (...)

Pipe::tcp_read() first calls Pipe::tcp_read_wait() to check if data is
avaliable. If there are prefetched data, Pipe::tcp_read_wait() return
immediately. Pipe::buffered_recv() is called, which reads data from the
prefetched data. If prefetched data isn't enough, Pipe::buffered_recv()
calls Pipe::do_recv() to read data from socket. But it's possble that
socket has no data at this time, so Pipe::do_recv() keeps retry.

The fix is simple, just not retry when recv(2) return -EAGAIN.

Fixes: #14120
Signed-off-by: Yan, Zheng <zyan@redhat.com>