Squashfs: optimize reading uncompressed data

When dealing with uncompressed data, there is no need to read a whole
block (default 128K) to get the desired page: the pages are
independent from each others.

This patch change the readpages logic so that reading uncompressed
data only read the number of pages advised by the readahead algorithm.

Moreover, if the page actor contains holes (i.e. pages that are already
up-to-date), squashfs skips the buffer_head associated to those pages.

This patch greatly improve the performance of random reads for
uncompressed files because squashfs only read what is needed. It also
reduces the number of unnecessary reads.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: I8ecb2f63f81fd798916074848d36c1a3d70cc5c1