The previous code acquired a read lock on src and a write lock on
dst for the entire duration of Splice. This resulted in deadlock,
in a situation akin to the following:

Splice is blocking, waiting to read from src.

The caller tries to close dst from another goroutine, but Close on
dst blocks in runtime.semacquire, because Splice is still holding a
write lock on it, and the Close didn't unblock any I/O.

The caller cannot unblock the read side of Splice through other means,
because they are stuck waiting in dst.Close().

Use more fine-grained locking instead: acquire the read lock on src
just before trying to splice from the source socket to the pipe,
and acquire the write lock on dst just before trying to splice from
the pipe to the destination socket.

Fixes #25985

Change-Id: I264c91c7a69bb6c5e28610e2bd801244804cf86d
Reviewed-on: https://go-review.googlesource.com/120317
Run-TryBot: Aram Hăvărneanu <aram@mgk.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>