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
211c17f51f46 ("Fix corners in writepage and btrfs_truncate_page"). The
latter is a leftover that should've been removed in 40f765805f08
("Btrfs: split up __extent_writepage to lower stack usage"). Remove
both.

Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Omar Sandoval <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/extent_io.c

···
 	u64 page_end = start + PAGE_SIZE - 1;
 	int ret;
 	int nr = 0;
-	size_t pg_offset = 0;
+	size_t pg_offset;
 	loff_t i_size = i_size_read(inode);
 	unsigned long end_index = i_size >> PAGE_SHIFT;
 	unsigned long nr_written = 0;
···
 		kunmap_atomic(userpage);
 		flush_dcache_page(page);
 	}
-
-	pg_offset = 0;
 
 	set_page_extent_mapped(page);