kernel.h: move PTR_IF() and u64_to_user_ptr() to util_macros.h

While the natural choice of PTR_IF() is kconfig.h, the latter is too broad
to include C code and actually the macro was moved out from there in the
past.  But kernel.h is neither a good choice for that.  Move it to
util_macros.h.  Do the same for u64_to_user_ptr().

While moving, add necessary documentation.

Link: https://lkml.kernel.org/r/20250324105228.775784-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexandru Ardelean <aardelean@baylibre.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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