btrfs: add dummy callback for readpage_io_failed and drop checks

Make extent_io_ops::readpage_io_failed_hook callback mandatory and
define a dummy function for btrfs_extent_io_ops. As the failed IO
callback is not performance critical, the branch vs extra trade off does
not hurt.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -4658,7 +4658,7 @@ static const struct extent_io_ops btree_extent_io_ops = {
 	.readpage_end_io_hook = btree_readpage_end_io_hook,
 	/* note we're sharing with inode.c for the merge bio hook */
 	.merge_bio_hook = btrfs_merge_bio_hook,
+	.readpage_io_failed_hook = btree_io_failed_hook,
 
 	/* optional callbacks */
-	.readpage_io_failed_hook = btree_io_failed_hook,
 };

fs/btrfs/extent_io.c

@@ -2581,7 +2581,7 @@ static void end_bio_extent_readpage(struct bio *bio)
 		if (likely(uptodate))
 			goto readpage_ok;
 
-		if (tree->ops && tree->ops->readpage_io_failed_hook) {
+		if (tree->ops) {
 			ret = tree->ops->readpage_io_failed_hook(page, mirror);
 			if (!ret && !bio->bi_error)
 				uptodate = 1;

fs/btrfs/extent_io.h

@@ -103,6 +103,7 @@ struct extent_io_ops {
 	int (*merge_bio_hook)(struct page *page, unsigned long offset,
 			      size_t size, struct bio *bio,
 			      unsigned long bio_flags);
+	int (*readpage_io_failed_hook)(struct page *page, int failed_mirror);
 
 	/*
 	 * Optional hooks, called if the pointer is not NULL
@@ -110,7 +111,6 @@ struct extent_io_ops {
 	int (*fill_delalloc)(struct inode *inode, struct page *locked_page,
 			     u64 start, u64 end, int *page_started,
 			     unsigned long *nr_written);
-	int (*readpage_io_failed_hook)(struct page *page, int failed_mirror);
 
 	int (*writepage_start_hook)(struct page *page, u64 start, u64 end);
 	void (*writepage_end_io_hook)(struct page *page, u64 start, u64 end,

fs/btrfs/inode.c

@@ -10509,6 +10509,12 @@ out_inode:
 
 }
 
+__attribute__((const))
+static int dummy_readpage_io_failed_hook(struct page *page, int failed_mirror)
+{
+	return 0;
+}
+
 static const struct inode_operations btrfs_dir_inode_operations = {
 	.getattr	= btrfs_getattr,
 	.lookup		= btrfs_lookup,
@@ -10551,6 +10557,7 @@ static const struct extent_io_ops btrfs_extent_io_ops = {
 	.submit_bio_hook = btrfs_submit_bio_hook,
 	.readpage_end_io_hook = btrfs_readpage_end_io_hook,
 	.merge_bio_hook = btrfs_merge_bio_hook,
+	.readpage_io_failed_hook = dummy_readpage_io_failed_hook,
 
 	/* optional callbacks */
 	.fill_delalloc = run_delalloc_range,