fs/ntfs3: improve readahead for bitmap initialization and large directory scans

fs/ntfs3: improve readahead for bitmap initialization and large directory scans

Previously sequential reads operations relied solely on single-page reads,
causing the block layer to perform many synchronous I/O requests,
especially for large volumes or large directories. This patch introduces
explicit readahead via page_cache_sync_readahead() and file_ra_state to
reduce I/O latency and improve sequential throughput.

Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>