net: fix sendfile for Solaris

If a retryable error such as EAGAIN/EINTR is encountered during a call
to sendfile(), we should not assume that a partial write occurred.
Instead, just like any other platform, we should always try again even
if 0 bytes were written.

Fixes #20857

Change-Id: I9aa48d193c27c6794c550320da4d3f7544041f37
Reviewed-on: https://go-review.googlesource.com/47351
Run-TryBot: Shawn Walker-Salas <shawn.walker@oracle.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/poll/sendfile_solaris.go

@@ -33,10 +33,7 @@ func SendFile(dstFD *FD, src int, pos, remain int64) (int64, error) {
 		n, err1 := syscall.Sendfile(dst, src, &pos1, n)
 		if err1 == syscall.EAGAIN || err1 == syscall.EINTR {
 			// partial write may have occurred
-			if n = int(pos1 - pos); n == 0 {
-				// nothing more to write
-				err1 = nil
-			}
+			n = int(pos1 - pos)
 		}
 		if n > 0 {
 			pos += int64(n)