EX-7601 ofd: convert dt_bufs_get to offset, len

EX-7601 ofd: convert dt_bufs_get to offset, len

dt_bufs_get takes a remote niobuf, but just uses the
offset and length for getting pages.

Compression requires rounding the local IO to include the
full compression chunk, which means the local IO does not
match the remote niobuf any more.

So we modify dt_bufs_get to take an offset and length
rather than a remote niobuf, so we can ask for the pages we
need.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I4beaf8207fa00d802c0a339df3de2a3c71154fc7
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52865
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>