Fixed incorrect range checks in mmap of framebuffer (kern/162373)

Implemented a fix for incorrect/invalid range checks in the vesa
framebuffer when attempting to mmap the framebuffer device. As
described in kern/162373, the original code contains a bug and will
incorrectly refuse to mmap at certain modes/resolutions because it
thinks the mmap length is out-of-range when it isn't.

Testing indicates that the revised framebuffer range check (mmap
offset is less than the window buffer size instead of the old check
that the offset is less than the window size minuse pagesize, which
really makes no sense) is the correct way to go.