btrfs: read eb folio index right before loops

btrfs: read eb folio index right before loops

There are generic helpers to access extent buffer folio data of any
length, potentially iterating over a few of them. This is a slow path,
either we use the type based accessors or the eb folio allocation is
contiguous and we can use the memcpy/memcmp helpers.

The initialization of 'i' is done at the beginning though it may not be
needed. Move it right before the folio loop, this has minor effect on
generated code in __write_extent_buffer().

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>