lib/iov_iter: Document _copy_to_iter_flushcache()

EXPORT_SYMBOL(_copy_from_iter_nocache);

#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE

/**

 * _copy_from_iter_flushcache - write destination through cpu cache

 * @addr: destination kernel address

 * @bytes: total transfer length

 * @iter: source iterator

 *

 * The pmem driver arranges for filesystem-dax to use this facility via

 * dax_copy_from_iter() for ensuring that writes to persistent memory

 * are flushed through the CPU cache. It is differentiated from

 * _copy_from_iter_nocache() in that guarantees all data is flushed for

 * all iterator types. The _copy_from_iter_nocache() only attempts to

 * bypass the cache for the ITER_IOVEC case, and on some archs may use

 * instructions that strand dirty-data in the cache.

 */

size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)

{

	char *to = addr;