btrfs: remove unnecessary pg_offset assignments in __extent_writepage()

We're initializing pg_offset to 0, setting it immediately, then
reassigning it to 0 again after. The former became unnecessary in
211c17f5 ("Fix corners in writepage and btrfs_truncate_page"). The
latter is a leftover that should've been removed in 40f76580
("Btrfs: split up __extent_writepage to lower stack usage"). Remove
both.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>