iomap: move the file mapping reporting code into a separate file

Move the file mapping reporting code (FIEMAP/FIBMAP) into a separate
file so that we can group related functions in a single file instead of
having a single enormous source file.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>