Fix up more fallout form alpha signal cleanups

Commit c52c2ddc ("alpha: switch osf_sigprocmask() to use of
sigprocmask()") had several problems.  The more obvious compile issues
got fixed in commit 0f44fbd2 ("alpha: fix compile problem in
arch/alpha/kernel/signal.c"), but it also caused a regression.

Since _BLOCKABLE is already the set of signals that can be blocked, the
code should do "newmask & _BLOCKABLE" rather than inverting _BLOCKABLE
before masking.

Reported-by: Michael Cree <mcree@orcon.net.nz>
Patch-by: Al Viro <viro@zeniv.linux.org.uk>
Patch-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>