Btrfs: fix typo in fallocate to make it honor actual size

There is a typo in __btrfs_prealloc_file_range() where we set the i_size to
actual_len/cur_offset, and then just set it to cur_offset again, and do the same
with btrfs_ordered_update_i_size().  This fixes it back to keeping i_size in a
local variable and then updating i_size properly.  Tested this with

xfs_io -F -f -c "falloc 0 1" -c "pwrite 0 1" foo

stat'ing foo gives us a size of 1 instead of 4096 like it was.  Thanks,
Signed-off-by: NJosef Bacik <josef@redhat.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -7002,6 +7002,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_key ins;
 	u64 cur_offset = start;
+	u64 i_size;
 	int ret = 0;
 	bool own_trans = true;
 
@@ -7043,11 +7044,11 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 		    (actual_len > inode->i_size) &&
 		    (cur_offset > inode->i_size)) {
 			if (cur_offset > actual_len)
-				i_size_write(inode, actual_len);
+				i_size = actual_len;
 			else
-				i_size_write(inode, cur_offset);
-			i_size_write(inode, cur_offset);
-			btrfs_ordered_update_i_size(inode, cur_offset, NULL);
+				i_size = cur_offset;
+			i_size_write(inode, i_size);
+			btrfs_ordered_update_i_size(inode, i_size, NULL);
 		}
 
 		ret = btrfs_update_inode(trans, root, inode);