avr32: use set_current_blocked() in handle_signal/sys_rt_sigreturn

It is wrong to change ->blocked directly, see e6fa16ab.  Change
handle_signal() and sys_rt_sigreturn() to use the right helper,
set_current_blocked().

Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Acked-by: Havard Skinnemoen <hskinnemoen@gmail.com>
Reviewed-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>