btrfs: rename variables clashing with global function names

reported by gcc -Wshadow:
page_index, page_offset, new_inode, dev_name

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index af2d717..b9ce2f7 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -164,7 +164,7 @@
 
 typedef struct extent_map *(get_extent_t)(struct inode *inode,
 					  struct page *page,
-					  size_t page_offset,
+					  size_t pg_offset,
 					  u64 start, u64 len,
 					  int create);