Bend zpl_set_acl to permit the new userns* parameter

Just like #12087, the set_acl signature changed with all the bolted-on
*userns parameters, which disabled set_acl usage, and caused #12076.

Turn zpl_set_acl into zpl_set_acl and zpl_set_acl_impl, and add a
new configure test for the new version.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12076
Closes #12093