xfs: rewrite getbmap using the xfs_iext_* helpers

Currently getbmap uses xfs_bmapi_read to query the extent map, and then
fixes up various bits that are eventually reported to userspace.

This patch instead rewrites it to use xfs_iext_lookup_extent and
xfs_iext_get_extent to iteratively process the extent map.  This not
only avoids the need to allocate a map for the returned xfs_bmbt_irec
structures but also greatly simplified the code.

There are two intentional behavior changes compared to the old code:

 - the current code reports unwritten extents that don't directly border
   a written one as unwritten even when not passing the BMV_IF_PREALLOC
   option, contrary to the documentation.  The new code requires the
   BMV_IF_PREALLOC flag to report the unwrittent extent bit.
 - The new code does never merges consecutive extents, unlike the old
   code that sometimes does it based on the boundaries of the
   xfs_bmapi_read calls.  Note that the extent merging behavior was
   entirely undocumented.

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