generic: add test to verify xattr replace operations are atomic

generic: add test to verify xattr replace operations are atomic

This test verifies that replacing a xattr's value is an atomic
operation. This is motivated by an issue in btrfs where replacing
a xattr's value wasn't an atomic operation, it consisted of
removing the old value and then inserting the new value in a
btree. This made readers (getxattr and listxattrs) not getting
neither the old nor the new value during a short time window.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>