Fix fallocate() test

Fix fallocate() test

The test for fallocate was broken because it used $TEST_DIR/$tmp.io. Because
$tmp is usually something like /tmp/1234 or /mnt/1234 the file cannot be
created and xfs_io fails regardless of existance of fallocate support. Moreover
the subsequent message parsing decides that fallocate is actually supported
because it does not expect this message.

Fix the test to not use $tmp.

Signed-off-by: Jan Kara <jack@suse.cz>