OpenZFS (since 5c0061345b824eebe7a6578528f873ffcaae1cdd) now reject
attribute names with certain prefixes.  This broke the Linux syscalls
compatibility wrappers that the test uses.  The FreeBSD syscalls use
separate arguments for the namespace and the attribute name, so the
passed attribute name shouldn't include a prefix like "user.".

The wrappers are only ever used with (and only support) the user
namespace, so just stripping the "user." prefix from the name fixes it.