introduce __block_write_begin

Split up the block_write_begin implementation - __block_write_begin is a new
trivial wrapper for block_prepare_write that always takes an already
allocated page and can be either called from block_write_begin or filesystem
code that already has a page allocated.  Remove the handling of already
allocated pages from block_write_begin after switching all callers that
do it to __block_write_begin.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/reiserfs/inode.c b/fs/reiserfs/inode.c
index 4c1fb54..045729f 100644
--- a/fs/reiserfs/inode.c
+++ b/fs/reiserfs/inode.c
@@ -2585,8 +2585,7 @@
 		old_ref = th->t_refcount;
 		th->t_refcount++;
 	}
-	ret = block_write_begin(file, mapping, pos, len, flags, pagep, fsdata,
-				reiserfs_get_block);
+	ret = __block_write_begin(page, pos, len, reiserfs_get_block);
 	if (ret && reiserfs_transaction_running(inode->i_sb)) {
 		struct reiserfs_transaction_handle *th = current->journal_info;
 		/* this gets a little ugly.  If reiserfs_get_block returned an