btrfs: cleanup, use enum values for btrfs_path reada

Replace the integers by enums for better readability. The value 2 does
not have any meaning since a717531942f488209dded30f6bc648167bcefa72
"Btrfs: do less aggressive btree readahead" (2009-01-22).

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/inode-map.c b/fs/btrfs/inode-map.c
index 53014e9..1951ad6 100644
--- a/fs/btrfs/inode-map.c
+++ b/fs/btrfs/inode-map.c
@@ -48,7 +48,7 @@
 	/* Since the commit root is read-only, we can safely skip locking. */
 	path->skip_locking = 1;
 	path->search_commit_root = 1;
-	path->reada = 2;
+	path->reada = READA_FORWARD;
 
 	key.objectid = BTRFS_FIRST_FREE_OBJECTID;
 	key.offset = 0;