Add xattr_handler support for Android kernels

Some ARM BSPs run the Android kernel, which has
a modified xattr_handler->get() function signature.
This adds support to compile against these kernels.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Walter Huf <hufman@gmail.com>
Closes #13824