Squashfs: implement .readpages()

Squashfs does not implement .readpages(), so the kernel just repeatedly
calls .readpage().

The readpages function tries to pack as much pages as possible in the
same page actor so that only 1 read request is issued.

Now that the read requests are asynchronous, the kernel can truly
prefetch pages using its readahead algorithm.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: I597b062cdde2f90048ed367c394ecc6750cdcf3c