MIPS: ftrace: Tweak safe_load()/safe_store() macros

Due to name collision in ftrace safe_load and safe_store macros,
these macros cannot take expressions as operands.

For example, compiler will complain for a macro call like the following:
  safe_store_code(new_code2, ip + 4, faulted);

  arch/mips/include/asm/ftrace.h:61:6: note: in definition of macro 'safe_store'
     : [dst] "r" (dst), [src] "r" (src)\
        ^
  arch/mips/kernel/ftrace.c:118:2: note: in expansion of macro 'safe_store_code'
    safe_store_code(new_code2, ip + 4, faulted);
    ^
  arch/mips/kernel/ftrace.c:118:32: error: undefined named operand 'ip + 4'
    safe_store_code(new_code2, ip + 4, faulted);
                                  ^
  arch/mips/include/asm/ftrace.h:61:6: note: in definition of macro 'safe_store'
     : [dst] "r" (dst), [src] "r" (src)\
        ^
  arch/mips/kernel/ftrace.c:118:2: note: in expansion of macro 'safe_store_code'
    safe_store_code(new_code2, ip + 4, faulted);
    ^

This build error is triggered by a4671094 [MIPS: ftrace: Fix icache flush
range error].  Tweak variable naming in those macros to allow flexible
operands.

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: rostedt@goodmis.org
Cc: fweisbec@gmail.com
Cc: mingo@redhat.com
Cc: Qais.Yousef@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/6622/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>