fix vfs/utils.c for big-endian systems

fix vfs/utils.c for big-endian systems

generic/633 was failing with EINVAL on the fsxgetattr call on s390.
Looks like this is due to a failure to properly endian swap the
arguments to the syscall, so fix that, and the magic_etc compare
in expected_dummy_vfs_caps_uid() as well while we're at it.

Fixes: 0d1af68e ("generic: add fstests for idmapped mounts")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>