xfs: fix iomap hole map reporting for zoned zero range

xfs: fix iomap hole map reporting for zoned zero range

The hole mapping logic for zero range in zoned mode is not quite
correct. It currently reports a hole whenever one exists in the data
fork. If the first write to a sparse range has completed and not yet
written back, the blocks exist in the COW fork as delalloc until
writeback completes, at which point they are allocated and mapped
into the data fork. If a zero range occurs on a range that has not
yet populated the data fork, we will incorrectly report it as a
hole.

Note that this currently functions correctly because we are bailed
out by the pagecache flush in iomap_zero_range(). If a hole or
unwritten mapping is reported with dirty pagecache, it assumes there
is pending data, flushes to induce any pending block
allocations/remaps, and retries the lookup. We want to remove this
hack from iomap, however, so update iomap_begin() to only report a
hole for zeroing when one exists in both forks.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>