xfs/649: fix various problems

xfs/649: fix various problems

Fix a couple of problems with this new test:

First, the comment for the $attr_size_bytes check says that the maximum
size of a single xattr value is 64k.  This is true, but the check
triggers the warning even when the size is exactly 64k.

Second, the test fails to format with 32k and 64k fsblock size
filesystems because the scsi_debug device size is 128M, and that's not
large enough for the minimum log size.  Raise that to 320M.

Cc: fstests@vger.kernel.org # v2025.12.09
Fixes: 5760b2105985bf ("xfs: test case for handling io errors when reading extended attributes")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>