EX-6127 llite: fix chunk_bits usage in readahead

EX-6127 llite: fix chunk_bits usage in readahead

For the minimum compression chunk size, chunk bits is zero,
so we cannot use if (chunk_bits) to determine if we're
doing compression.

This also fixes two other things:
1. A rounding error when rounding to chunk
2. Move rounding of end_idx to before first usage of
end_idx, so calculation of number of pages is correct
Without this, when the user reads 1 page or less, readahead
will calculate the readahead page count as 0 and will exit
without reading the chunk.

Fixes: c05d5990f4 ("EX-6127 llite: getting stripe info optimization")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I273506fd4f6ed5f0b8b5020357fd7caf0531e61c
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51504
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>