avr32: use block_sigmask()

Use the new helper function introduced in commit 5e6292c0 ("signal:
add block_sigmask() for adding sigmask to current->blocked") which
centralises the code for updating current->blocked after successfully
delivering a signal and reduces the amount of duplicate code across
architectures.

In the past some architectures got this code wrong, so using this helper
function should stop that from happening again.

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