console: limit the range of VGACON_SOFT_SCROLLBACK_SIZE

BuraphaLinux reported that we will trigger a mm warning when we
CONFIG_VGACON_SOFT_SCROLLBACK_SIZE=65536, this is because mm cann't
allocate so many pages.  We should limit the range of
CONFIG_VGACON_SOFT_SCROLLBACK_SIZE, don't give a user any chance to
trigger that.

Reported-by: BuraphaLinux Server <buraphalinuxserver@gmail.com>
Tested-by: BuraphaLinux Server <buraphalinuxserver@gmail.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>