vdso: Fix powerpc build U64_MAX undeclared error

U64_MAX is not in include/vdso/limits.h, although that isn't noticed on x86
because x86 includes include/linux/limits.h indirectly. However powerpc is
more selective, resulting in the following build error:

  In file included from <command-line>:
  lib/vdso/gettimeofday.c: In function 'vdso_calc_ns':
  lib/vdso/gettimeofday.c:11:33: error: 'U64_MAX' undeclared
     11 | # define VDSO_DELTA_MASK(vd)    U64_MAX
        |                                 ^~~~~~~

Use ULLONG_MAX instead which will work just as well and is in
include/vdso/limits.h.

Fixes: c8e3a8b ("vdso: Consolidate vdso_calc_delta()")
Reported-by: Stephen Rothwell <[email protected]>
Signed-off-by: Adrian Hunter <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Closes: https://lore.kernel.org/all/[email protected]/

Author: ahunter6

lib/vdso/gettimeofday.c

@@ -8,7 +8,7 @@
 #ifndef vdso_calc_ns
 
 #ifdef VDSO_DELTA_NOMASK
-# define VDSO_DELTA_MASK(vd)	U64_MAX
+# define VDSO_DELTA_MASK(vd)	ULLONG_MAX
 #else
 # define VDSO_DELTA_MASK(vd)	(vd->mask)
 #endif