nobh: nobh_write_end fix

This path mustn't have been tested :( I did attempt to exercise it
by injecting failures here, but I suspect PageMappedToDisk may have
been getting in the way. Will need more of a look, although I think
nobh mode is OK for an -rc1 (it shouldn't eat anyone's data).

Commit 03158cd7 ("fs: restore nobh")
introcduced a NULL deref.  Spotted by the Coverity checker.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/buffer.c

@@ -2563,7 +2563,7 @@ int nobh_write_end(struct file *file, struct address_space *mapping,
 			struct page *page, void *fsdata)
 {
 	struct inode *inode = page->mapping->host;
-	struct buffer_head *head = NULL;
+	struct buffer_head *head = fsdata;
 	struct buffer_head *bh;
 
 	if (!PageMappedToDisk(page)) {
@@ -2584,7 +2584,6 @@ int nobh_write_end(struct file *file, struct address_space *mapping,
 	unlock_page(page);
 	page_cache_release(page);
 
-	head = fsdata;
 	while (head) {
 		bh = head;
 		head = head->b_this_page;