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/compression.h b/fs/btrfs/compression.h
index 5100017..a12059f 100644
--- a/fs/btrfs/compression.h
+++ b/fs/btrfs/compression.h
@@ -37,7 +37,7 @@
 int btrfs_decompress_buf2page(char *buf, unsigned long buf_start,
 			      unsigned long total_out, u64 disk_start,
 			      struct bio_vec *bvec, int vcnt,
-			      unsigned long *page_index,
+			      unsigned long *pg_index,
 			      unsigned long *pg_offset);
 
 int btrfs_submit_compressed_write(struct inode *inode, u64 start,