fs: kill block_prepare_write

__block_write_begin and block_prepare_write are identical except for slightly
different calling conventions.  Convert all callers to the __block_write_begin
calling conventions and drop block_prepare_write.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
index 6b24afb..4f36f88 100644
--- a/fs/gfs2/aops.c
+++ b/fs/gfs2/aops.c
@@ -618,7 +618,6 @@
 	struct gfs2_alloc *al = NULL;
 	pgoff_t index = pos >> PAGE_CACHE_SHIFT;
 	unsigned from = pos & (PAGE_CACHE_SIZE - 1);
-	unsigned to = from + len;
 	struct page *page;
 
 	gfs2_holder_init(ip->i_gl, LM_ST_EXCLUSIVE, 0, &ip->i_gh);
@@ -691,7 +690,7 @@
 	}
 
 prepare_write:
-	error = block_prepare_write(page, from, to, gfs2_block_map);
+	error = __block_write_begin(page, from, len, gfs2_block_map);
 out:
 	if (error == 0)
 		return 0;