btrfs: split btrfs_is_fsstree() into multiple if statements for readability

Instead of a single if statement with multiple conditions, split it into
several if statements testing only one condition at a time and return true
or false immediately after. This makes it more immediate to reason.

The module's text size also slightly decreases, at least with gcc 14.2.0
on x86_64.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1897138	 161583	  16136	2074857	 1fa8e9	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1896976	 161583	  16136	2074695	 1fa847	fs/btrfs/btrfs.ko

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ctree.h

@@ -723,11 +723,16 @@ int btrfs_leaf_free_space(const struct extent_buffer *leaf);
 
 static inline bool btrfs_is_fstree(u64 rootid)
 {
-	if (rootid == BTRFS_FS_TREE_OBJECTID ||
-	    ((s64)rootid >= (s64)BTRFS_FIRST_FREE_OBJECTID &&
-	      !btrfs_qgroup_level(rootid)))
+	if (rootid == BTRFS_FS_TREE_OBJECTID)
 		return true;
-	return false;
+
+	if ((s64)rootid < (s64)BTRFS_FIRST_FREE_OBJECTID)
+		return false;
+
+	if (btrfs_qgroup_level(rootid) != 0)
+		return false;
+
+	return true;
 }
 
 static inline bool btrfs_is_data_reloc_root(const struct btrfs_root *root)