checked_file: preserve DMA alignment
checked_file: preserve DMA alignment

Inherit the alignment parameters from the underlying file instead of
defaulting to 4096.  This gives better read performance on disks with 512-byte
sectors.

Fixes #1532.
Message-Id: <427cbbd5eaab99e6f4586e013bb20a7cb4a4501e@scylladb.com>

scylladb/scylla,duarten/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,kjniemi/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla