x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

Before we rework the "pmem api" to stop abusing __copy_user_nocache()
for memcpy_to_pmem() we need to fix cases where we may strand dirty data
in the cpu cache. The problem occurs when copy_from_iter_pmem() is used
for arbitrary data transfers from userspace. There is no guarantee that
these transfers, performed by dax_iomap_actor(), will have aligned
destinations or aligned transfer lengths. Backstop the usage
__copy_user_nocache() with explicit cache management in these unaligned
cases.

Yes, copy_from_iter_pmem() is now too big for an inline, but addressing
that is saved for a later patch that moves the entirety of the "pmem
api" into the pmem driver directly.

Fixes: 5de490daec8b ("pmem: add copy_from_iter_pmem() and clear_pmem()")
Cc: <[email protected]>
Cc: <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jeff Moyer <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Signed-off-by: Toshi Kani <[email protected]>
Signed-off-by: Dan Williams <[email protected]>