ext2fs: add readahead method to improve scanning

ext2fs: add readahead method to improve scanning

Add a readahead method for prefetching ranges of disk blocks.
This is useful for inode table scanning, and other large
contiguous ranges of blocks, and may also prove useful for
random block prefetch, since it will allow reordering of the
IO without waiting synchronously for the reads to complete.

It is currently using the posix_fadvise(POSIX_FADV_WILLNEED)
interface, as this proved most efficient during our testing

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>