btrfs: use variable for end offset in extent_writepage_io()

btrfs: use variable for end offset in extent_writepage_io()

[ Upstream commit 46a23908598f4b8e61483f04ea9f471b2affc58a ]

Instead of repeating the expression "start + len" multiple times, store it
in a variable and use it where needed.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: e9e3b22ddfa7 ("btrfs: fix beyond-EOF write handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>