alpha: Use set_current_blocked() and block_sigmask()

alpha: Use set_current_blocked() and block_sigmask()

As described in e6fa16ab ("signal: sigprocmask() should do
retarget_shared_pending()") the modification of current->blocked is
incorrect as we need to check for shared signals we're about to block.

Also, use the new helper function block_sigmask() 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.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>