seccomp: refactor line handling of parse_config

seccomp: refactor line handling of parse_config

Moving parse_config_v2 to use getline accidentally parsed
the wrong buffer. Since both _v1 and _v2 now use getline it
seems to be simpler to also use getline() for the first line
before entering the version specific parsers and pass along
the pointer and size so they can reuse them.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Fixes: 9c3798eba41c ("seccomp: parse_config_v2()")