xfs: don't assume preallocation is always supported on XFS

xfs: don't assume preallocation is always supported on XFS

The always_cow mode can't usefull preallocate space gіven that it
always has to write out of place, and thus will reject falloc or
ioctl calls to preallocate space in a file.  Add explicit checks for
preallocation support in various XFS-specific tests to support this.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>