generic/772: actually check for file_getattr special file support

On XFS in 6.17, this test fails with:

 --- /run/fstests/bin/tests/generic/772.out	2025-10-06 08:27:10.834318149 -0700
 +++ /var/tmp/fstests/generic/772.out.bad	2025-10-08 18:00:34.713388178 -0700
 @@ -9,29 +9,34 @@ Can not get fsxattr on ./foo: Invalid ar
  Can not set fsxattr on ./foo: Invalid argument
  Initial attributes state
  ----------------- SCRATCH_MNT/prj
 ------------------ ./fifo
 ------------------ ./chardev
 ------------------ ./blockdev
 ------------------ ./socket
 ------------------ ./foo
 ------------------ ./symlink
 +Can not get fsxattr on ./fifo: Inappropriate ioctl for device
 +Can not get fsxattr on ./chardev: Inappropriate ioctl for device
 +Can not get fsxattr on ./blockdev: Inappropriate ioctl for device
 +Can not get fsxattr on ./socket: Inappropriate ioctl for device

This is a result of XFS' file_getattr implementation rejecting special
files prior to 6.18.  Therefore, skip this new test on old kernels.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>