xfs: test that truncate does not spuriously return ENOSPC

xfs: test that truncate does not spuriously return ENOSPC

For zoned file systems, truncate to an offset not aligned to the block
size need to allocate a new block for zeroing the remainder.

Test that this allocation can dip into the reserved pool even when other
threads are waiting for space freed by GC.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>