ZTS: Fix xattr_004_pos failure, don't use tmpfs

Previously, xattr_004_pos would create files with xattrs on both
tmpfs and ext2, and then copy them to zfs to verify that their
xattrs were preserved. However tmpfs doesn't support xattrs.

This was never noticed until Fedora 33. In Fedora 32 and older,
/tmp was on the root partition (like ext4), whereas on Fedora 33
/tmp is actually tmpfs. That caused this test to fail on Fedora 33.

This fix updates the test to only create the file on ext2, not tmpfs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #11133