zlib: Rename write variable to wnext (window write index)

There is no particular patch/description which described the reason for
this change but it was done as the part of zlib 1.2.3.5 release done by
zlib commit d004b047838a ("zlib 1.2.3.5"). It is preparation for followup
patch.

Signed-off-by: Michal Simek <michal.simek@amd.com>