btrfs: make the rule checking more readable for should_cow_block()

It's quite hard and unreadable the way the rule checks are organized in
should_cow_block(). We have a single if statement that returns 0 (false)
and it checks several conditions, with one them being a negated compound
condition which is particularly hard to reason immediately.

Improve on this by using multiple if statements, each checking a single
condition and returning immediately. Also change the return type from an
integer to a boolean, since all we need is to return true or false.

At least on x86_64 with Debian's gcc 14.2.0-19, this also reduces the
object code size by 64 bytes.

Before this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1913327	 161567	  15592	2090486	 1fe5f6	fs/btrfs/btrfs.ko

After this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1913263	 161567	  15592	2090422	 1fe5b6	fs/btrfs/btrfs.ko

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

Author: fdmanana

fs/btrfs/ctree.c

@@ -613,15 +613,12 @@ int btrfs_force_cow_block(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
-static inline int should_cow_block(const struct btrfs_trans_handle *trans,
-				   const struct btrfs_root *root,
-				   const struct extent_buffer *buf)
+static inline bool should_cow_block(const struct btrfs_trans_handle *trans,
+				    const struct btrfs_root *root,
+				    const struct extent_buffer *buf)
 {
 	if (btrfs_is_testing(root->fs_info))
-		return 0;
-
-	/* Ensure we can see the FORCE_COW bit */
-	smp_mb__before_atomic();
+		return false;
 
 	/*
 	 * We do not need to cow a block if
@@ -634,13 +631,25 @@ static inline int should_cow_block(const struct btrfs_trans_handle *trans,
 	 *    after we've finished copying src root, we must COW the shared
 	 *    block to ensure the metadata consistency.
 	 */
-	if (btrfs_header_generation(buf) == trans->transid &&
-	    !btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN) &&
-	    !(btrfs_root_id(root) != BTRFS_TREE_RELOC_OBJECTID &&
-	      btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC)) &&
-	    !test_bit(BTRFS_ROOT_FORCE_COW, &root->state))
-		return 0;
-	return 1;
+
+	if (btrfs_header_generation(buf) != trans->transid)
+		return true;
+
+	if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN))
+		return true;
+
+	/* Ensure we can see the FORCE_COW bit. */
+	smp_mb__before_atomic();
+	if (test_bit(BTRFS_ROOT_FORCE_COW, &root->state))
+		return true;
+
+	if (btrfs_root_id(root) == BTRFS_TREE_RELOC_OBJECTID)
+		return false;
+
+	if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC))
+		return true;
+
+	return false;
 }
 
 /*