btrfs: set EXTENT_NORESERVE bits side btrfs_dirty_pages()

Set the extent bits EXTENT_NORESERVE inside btrfs_dirty_pages() as
opposed to calling set_extent_bits again later.

Fold check for written length within the function.

Note: EXTENT_NORESERVE is set before unlocking extents.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>