fbmem: fix con2fbmap limit check

Fix limit check in FBIOPUT_CON2FBMAP ioctl.

Signed-off-by: Peter Samuelson <peter@p12n.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>