btrfs: call submit_bio_hook directly for metadata pages

No need to go through a function pointer indirection simply call
submit_bio_hook directly by exporting and renaming the helper to
btrfs_submit_metadata_bio. This makes the code more readable and should
result in somewhat faster code due to no longer paying the price for
specualtive attack mitigations that come with indirect function calls.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -814,9 +814,8 @@ static int check_async_write(struct btrfs_fs_info *fs_info,
 	return 1;
 }
 
-static blk_status_t btree_submit_bio_hook(struct inode *inode, struct bio *bio,
-					  int mirror_num,
-					  unsigned long bio_flags)
+blk_status_t btrfs_submit_metadata_bio(struct inode *inode, struct bio *bio,
+				       int mirror_num, unsigned long bio_flags)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 	int async = check_async_write(fs_info, BTRFS_I(inode));
@@ -4636,6 +4635,5 @@ static int btrfs_cleanup_transaction(struct btrfs_fs_info *fs_info)
 }
 
 static const struct extent_io_ops btree_extent_io_ops = {
-	/* mandatory callbacks */
-	.submit_bio_hook = btree_submit_bio_hook,
+	.submit_bio_hook = NULL
 };

fs/btrfs/disk-io.h

@@ -79,6 +79,8 @@ void btrfs_drop_and_free_fs_root(struct btrfs_fs_info *fs_info,
 int btrfs_validate_metadata_buffer(struct btrfs_io_bio *io_bio, u64 phy_offset,
 				   struct page *page, u64 start, u64 end,
 				   int mirror);
+blk_status_t btrfs_submit_metadata_bio(struct inode *inode, struct bio *bio,
+				       int mirror_num, unsigned long bio_flags);
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 struct btrfs_root *btrfs_alloc_dummy_root(struct btrfs_fs_info *fs_info);
 #endif

fs/btrfs/extent_io.c

@@ -172,8 +172,8 @@ int __must_check submit_one_bio(struct bio *bio, int mirror_num,
 		ret = btrfs_submit_data_bio(tree->private_data, bio, mirror_num,
 					    bio_flags);
 	else
-		ret = tree->ops->submit_bio_hook(tree->private_data, bio,
-						 mirror_num, bio_flags);
+		ret = btrfs_submit_metadata_bio(tree->private_data, bio,
+						mirror_num, bio_flags);
 
 	return blk_status_to_errno(ret);
 }