Squashfs: replace buffer_head with BIO

The 'll_rw_block' has been deprecated and BIO is now the basic container
for block I/O within the kernel.

Switching to BIO offers 2 advantages:
  1/ It removes synchronous wait for the up-to-date buffers: SquashFS
     now deals with decompressions/copies asynchronously.
     Implementing an asynchronous mechanism to read data is needed to
     efficiently implement .readpages().
  2/ Prior to this patch, merging the read requests entirely depends on
     the IO scheduler. SquashFS has more information than the IO
     scheduler about what could be merged. Moreover, merging the reads
     at the FS level means that we rely less on the IO scheduler.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: I775d2e11f017476e1899518ab52d9d0a8a0bce28