Auto-port 5.0: Fix limit of bytes for gathering writes being ignored after the first buffer - #18404

… buffer (#18396)

Motivation:

netty_unix_filedescriptor_writev(...) stops adding buffers to a
gathering
write once their size exceeds maxBytesToWrite. The first buffer is
always
added, even if it is bigger than the limit. The comparison
`bytesLength > maxBytesToWrite` compared a size_t with a jlong, so the
jlong was converted to unsigned. After the first buffer was bigger than
maxBytesToWrite, the remaining value became negative and so was
converted
to a huge unsigned value. The check then never triggered again, and all
the remaining buffers (up to IOV_MAX) were added to the writev(...)
call.
For example, with a limit of 64KB and a first buffer of 1MB, all the
buffers that follow were included as well, which ignores the limit that
is used to adapt the size of gathering writes.

Modification:

Compare the length as signed jlong, so a negative remaining value is
handled correctly. This is done in all four variants of the loop.

Result:

The limit of bytes for a gathering write is respected, also if the first
buffer is bigger than the limit.

(cherry picked from commit 1ed8174)