asm-generic: percpu: avoid Wshadow warning

Nesting macros that use the same local variable names causes
warnings when building with "make W=2":

include/asm-generic/percpu.h:117:14: warning: declaration of '__ret' shadows a previous local [-Wshadow]
include/asm-generic/percpu.h:126:14: warning: declaration of '__ret' shadows a previous local [-Wshadow]

These are fairly harmless, but since the warning comes from
a global header, the warning happens every time the headers
are included, which is fairly annoying.

Rename the variables to avoid shadowing and shut up the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>