BUG/CRITICAL: http: don't update msg->sov once data start to leave the buffer

Commit bb2e669 ("BUG/MAJOR: http: correctly rewind the request body
after start of forwarding") was incorrect/incomplete. It used to rely on
CF_READ_ATTACHED to stop updating msg->sov once data start to leave the
buffer, but this is unreliable because since commit a6eebb3 ("[BUG]
session: clear BF_READ_ATTACHED before next I/O") merged in 1.5-dev1,
this flag is only ephemeral and is cleared once all analysers have
seen it. So we can start updating msg->sov again each time we pass
through this place with new data. With a sufficiently large amount of
data, it is possible to make msg->sov wrap and validate the if()
condition at the top, causing the buffer to advance by about 2GB and
crash the process.

Note that the offset cannot be controlled by the attacker because it is
a sum of millions of small random sizes depending on how many bytes were
read by the server and how many were left in the buffer, only because
of the speed difference between reading and writing. Also, nothing is
written, the invalid pointer resulting from this operation is only read.

Many thanks to James Dempsey for reporting this bug and to Chris Forbes for
narrowing down the faulty area enough to make its root cause analysable.

This fix must be backported to haproxy 1.5.
(cherry picked from commit a2d002aeb669a9bbca2dcd3e6be71615f435e9e6)
[RELEASE] Released version 1.5.4

Released version 1.5.4 with the following main changes :
    - BUG: config: error in http-response replace-header number of arguments
    - BUG/MINOR: Fix search for -p argument in systemd wrapper.
    - BUG/MEDIUM: auth: fix segfault with http-auth and a configuration with an unknown encryption algorithm
    - BUG/MEDIUM: config: userlists should ensure that encrypted passwords are supported
    - MEDIUM: connection: add new bit in Proxy Protocol V2
    - BUG/MINOR: server: move the directive #endif to the end of file
    - BUG/MEDIUM: http: tarpit timeout is reset
    - BUG/MAJOR: tcp: fix a possible busy spinning loop in content track-sc*
    - BUG/MEDIUM: http: fix inverted condition in pat_match_meth()
    - BUG/MEDIUM: http: fix improper parsing of HTTP methods for use with ACLs
    - BUG/MINOR: pattern: remove useless allocation of unused trash in pat_parse_reg()
    - BUG/MEDIUM: acl: correctly compute the output type when a converter is used
    - CLEANUP: acl: cleanup some of the redundancy and spaghetti after last fix
    - BUG/CRITICAL: http: don't update msg->sov once data start to leave the buffer