btrfs: make the logic from btrfs_block_can_be_shared() easier to read

The logic in btrfs_block_can_be_shared() is hard to follow as we have a
lot of conditions in a single if statement including a subexpression with
a logical or and two nested if statements inside the main if statement.

Make this easier to read by using separate if statements that return
immediately when we find a condition that determines if a block can be
or can not be shared.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -374,27 +374,35 @@ bool btrfs_block_can_be_shared(struct btrfs_trans_handle *trans,
 			       struct btrfs_root *root,
 			       struct extent_buffer *buf)
 {
+	const u64 buf_gen = btrfs_header_generation(buf);
+
 	/*
 	 * Tree blocks not in shareable trees and tree roots are never shared.
 	 * If a block was allocated after the last snapshot and the block was
 	 * not allocated by tree relocation, we know the block is not shared.
 	 */
-	if (test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
-	    buf != root->node &&
-	    (btrfs_header_generation(buf) <=
-	     btrfs_root_last_snapshot(&root->root_item) ||
-	     btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC))) {
-		if (buf != root->commit_root)
-			return true;
-		/*
-		 * An extent buffer that used to be the commit root may still be
-		 * shared because the tree height may have increased and it
-		 * became a child of a higher level root. This can happen when
-		 * snapshotting a subvolume created in the current transaction.
-		 */
-		if (btrfs_header_generation(buf) == trans->transid)
-			return true;
-	}
+
+	if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
+		return false;
+
+	if (buf == root->node)
+		return false;
+
+	if (buf_gen > btrfs_root_last_snapshot(&root->root_item) &&
+	    !btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC))
+		return false;
+
+	if (buf != root->commit_root)
+		return true;
+
+	/*
+	 * An extent buffer that used to be the commit root may still be shared
+	 * because the tree height may have increased and it became a child of a
+	 * higher level root. This can happen when snapshotting a subvolume
+	 * created in the current transaction.
+	 */
+	if (buf_gen == trans->transid)
+		return true;
 
 	return false;
 }