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/ctree.h b/fs/btrfs/ctree.h
index 8f4b81d..b5433bb 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2534,7 +2534,7 @@
 int btrfs_commit_write(struct file *file, struct page *page,
 		       unsigned from, unsigned to);
 struct extent_map *btrfs_get_extent(struct inode *inode, struct page *page,
-				    size_t page_offset, u64 start, u64 end,
+				    size_t pg_offset, u64 start, u64 end,
 				    int create);
 int btrfs_update_inode(struct btrfs_trans_handle *trans,
 			      struct btrfs_root *root,