Avoid reusing pending buffer; append incoming data instead

When reading from a stream, a single read may return only part of a
message segment. In such cases, the partial data was stored in
'iofd->pending' and reused for subsequent reads to complete the message.

With upcoming changes that submit multiple read SQEs to io_uring,
each read uses its own pre-submitted buffer. Reusing 'iofd->pending' for
submitting next read is not possible, as the next read buffer is already
submitted.

Instead, create a new msgb which is used for the read operation and,
once completed, memcopy to the existing pending buffer, allowing message
segments to accumulate until complete.

Change-Id: I08df9736ccc5e9a7df61ca6dcf94629ee010752f
---
M src/core/osmo_io.c
M src/core/osmo_io_internal.h
M src/core/osmo_io_poll.c
M src/core/osmo_io_uring.c
4 files changed, 26 insertions(+), 28 deletions(-)