arm64: percpu: Initialize ret in the default case

Clang warns that if the default case is taken, ret will be
uninitialized.

./arch/arm64/include/asm/percpu.h:196:2: warning: variable 'ret' is used
uninitialized whenever switch default is taken
[-Wsometimes-uninitialized]
        default:
        ^~~~~~~
./arch/arm64/include/asm/percpu.h:200:9: note: uninitialized use occurs
here
        return ret;
               ^~~
./arch/arm64/include/asm/percpu.h:157:19: note: initialize the variable
'ret' to silence this warning
        unsigned long ret, loop;
                         ^
                          = 0

This warning appears several times while building the erofs filesystem.
While it's not strictly wrong, the BUILD_BUG will prevent this from
becoming a true problem. Initialize ret to 0 in the default case right
before the BUILD_BUG to silence all of these warnings.

Reported-by: Prasad Sodagudi <psodagud@codeaurora.org>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>