target/mips: Fix PageMask with variable page size

target/mips: Fix PageMask with variable page size

Our current code assumed the target page size is always 4k
when handling PageMask and VPN2, however, variable page size
was just added to mips target and that's no longer true.

Fixes: ee3863b9d414 ("target/mips: Support variable page size")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <1604636510-8347-2-git-send-email-chenhc@lemote.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[PMD: Replaced find_first_zero_bit() by cto32()]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>