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: 35fb9ae4aa2e ("usercopy: Cast pointer to an integer once")
Cc: Matthew Wilcox <[e-mail]>
Cc: Uladzislau Rezki <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Joe Perches <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

mm/usercopy.c

···
 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;