mm: rename _count, field of the struct page, to _refcount

Many developers already know that field for reference count of the
struct page is _count and atomic type.  They would try to handle it
directly and this could break the purpose of page reference count
tracepoint.  To prevent direct _count modification, this patch rename it
to _refcount and add warning message on the code.  After that, developer
who need to handle reference count will find that field should not be
accessed directly.

[[email protected]: fix comments, per Vlastimil]
[[email protected]: Documentation/vm/transhuge.txt too]
[[email protected]: sync ethernet driver changes]
Signed-off-by: Joonsoo Kim <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Sunil Goutham <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Manish Chopra <[email protected]>
Cc: Yuval Mintz <[email protected]>
Cc: Tariq Toukan <[email protected]>
Cc: Saeed Mahameed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/hwtracing/intel_th/msu.c

@@ -1164,7 +1164,7 @@ static void msc_mmap_close(struct vm_area_struct *vma)
 	if (!atomic_dec_and_mutex_lock(&msc->mmap_count, &msc->buf_mutex))
 		return;
 
-	/* drop page _counts */
+	/* drop page _refcounts */
 	for (pg = 0; pg < msc->nr_pages; pg++) {
 		struct page *page = msc_buffer_get_page(msc, pg);