usercopy: use unsigned long instead of uintptr_t

A recent commit factored out a series of annoying (unsigned long) casts
into a single variable declaration, but made the pointer type a
`uintptr_t` rather than the usual `unsigned long`. This patch changes it
to be the integer type more typically used by the kernel to represent
addresses.

Fixes: 35fb9ae4 ("usercopy: Cast pointer to an integer once")
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220616143617.449094-1-Jason@zx2c4.com

mm/usercopy.c

@@ -161,7 +161,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 static inline void check_heap_object(const void *ptr, unsigned long n,
 				     bool to_user)
 {
-	uintptr_t addr = (uintptr_t)ptr;
+	unsigned long addr = (unsigned long)ptr;
 	unsigned long offset;
 	struct folio *folio;