nilfs2: add iomap operations for direct I/O

The patch adds iomap.h with declaration of iomaps
operations and iomap.c with implementation of
nilfs_iomap_begin(). This method is based on calling
the n

nilfs2: add iomap operations for direct I/O

The patch adds iomap.h with declaration of iomaps
operations and iomap.c with implementation of
nilfs_iomap_begin(). This method is based on calling
the nilfs_bmap_lookup_contig() method. It resolves
the file's block-mapping to find the sequence of
physically-contiguous blocks starting at starting
block. Offsets at or beyond i_size are reported
as a hole. For offsets within i_size, the requested
length is first clamped to the block-aligned end of the file.
Both the hole and mapped cases then report iomap->offset
aligned to the containing block (blkoff << i_blkbits)
rather than the raw request offset.

Co-developed-by: Linmao Li <lilinmao@kylinos.cn>
Signed-off-by: Linmao Li <lilinmao@kylinos.cn>
cc: Christoph Hellwig <hch@lst.de>
cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
cc: linux-nilfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>

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