xfs: test iomap zero range via fsx and error tag

xfs: test iomap zero range via fsx and error tag

iomap supports a zero range operation based on buffered writes. This
mechanism is used in limited spots, such as partial block zeroing,
etc., because usually for larger zeroing ops it is more efficient to
punch a hole and allocate unwritten extents.

This means iomap zero range has limited production test coverage
even though it has some particular corner cases that warrant test
coverage. XFS supports an error injection knob to force use of iomap
zero range on fallocate zero range operations. Add a test to run fsx
with this knob enabled to provide more zeroing test coverage.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>