vfb: make virtual framebuffer mmapable

Changed things:

1. vmalloc()/vfree() replaced with rvmalloc()/rvfree() (taken from
   drivers/media/video/se401.c)

2. mmap method implemented (mostly taken from drivers/media/video/se401.c)

3. smem_start and smem_len fields of struct fb_fix_screeninfo initialized.
    (smem_start initialized with virtual address, don't know if it is really
   bad...)

[adaplas: sparse warning fix]
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>