LoongArch: Add __percpu annotation for __percpu_read()/__percpu_write()

When build kernel with C=1, we get:

arch/loongarch/kernel/process.c:234:46: warning: incorrect type in argument 1 (different address spaces)
arch/loongarch/kernel/process.c:234:46:    expected void *ptr
arch/loongarch/kernel/process.c:234:46:    got unsigned long [noderef] __percpu *
arch/loongarch/kernel/process.c:234:46: warning: incorrect type in argument 1 (different address spaces)
arch/loongarch/kernel/process.c:234:46:    expected void *ptr
arch/loongarch/kernel/process.c:234:46:    got unsigned long [noderef] __percpu *
arch/loongarch/kernel/process.c:234:46: warning: incorrect type in argument 1 (different address spaces)
arch/loongarch/kernel/process.c:234:46:    expected void *ptr
arch/loongarch/kernel/process.c:234:46:    got unsigned long [noderef] __percpu *
arch/loongarch/kernel/process.c:234:46: warning: incorrect type in argument 1 (different address spaces)
arch/loongarch/kernel/process.c:234:46:    expected void *ptr
arch/loongarch/kernel/process.c:234:46:    got unsigned long [noderef] __percpu *

Add __percpu annotation for __percpu_read()/__percpu_write() can avoid
such warnings. __percpu_xchg() and other functions don't need annotation
because their wrapper, i.e. _pcp_protect(), already suppresses warnings.

Also adjust the indentations in this file.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311080409.LlOfTR3m-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311080840.Vc2kXhfp-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311081340.3k72KKdg-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311120926.cjYHyoYw-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311152142.g6UyNx1R-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311160339.DbhaH8LX-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202311181454.CTPrSYmQ-lkp@intel.com/
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>