uio, lib: Fix CONFIG_ARCH_HAS_UACCESS_MCSAFE compilation

Add a common Kconfig CONFIG_ARCH_HAS_UACCESS_MCSAFE that archs can
optionally select, and fixup the declaration of _copy_to_iter_mcsafe().

Fixes: 8780356e ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

include/linux/uio.h

@@ -155,7 +155,7 @@ size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i);
 #endif
 
 #ifdef CONFIG_ARCH_HAS_UACCESS_MCSAFE
-size_t _copy_to_iter_mcsafe(void *addr, size_t bytes, struct iov_iter *i);
+size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i);
 #else
 #define _copy_to_iter_mcsafe _copy_to_iter
 #endif

lib/Kconfig

@@ -586,6 +586,9 @@ config ARCH_HAS_PMEM_API
 config ARCH_HAS_UACCESS_FLUSHCACHE
 	bool
 
+config ARCH_HAS_UACCESS_MCSAFE
+	bool
+
 config STACKDEPOT
 	bool
 	select STACKTRACE