BUG/MINOR: quic: Possible unexpected counter incrementation on send*() errors

BUG/MINOR: quic: Possible unexpected counter incrementation on send*() errors

Some counters could potentially be incremented even if send*() syscall returned
no error when ret >= 0 and ret != sz. This could be the case for instance if
a first call to send*() returned -1 with errno set to EINTR (or any previous syscall
which set errno to a non-null value) and if the next call to send*() returned
something positive and smaller than <sz>.

Must be backported to 2.7 and 2.6.