Add missing NULL in zpl_xattr_handlers

The xattr_resolve_name() helper function expects the registered
list of xattr handlers to be NULL terminated. This NULL was
accidentally missing which could result in a NULL dereference.

Interestingly this issue only manifested itself on certain 32-bit
systems. Presumably on 64-bit kernels we just always happen to
get lucky and the memory following the structure is zeroed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #594