bcachefs: Use filemap_read() to simplify the execution flow

Using filemap_read() can reduce unnecessary code execution
for non IOCB_DIRECT paths.
Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io-direct.c

@@ -179,7 +179,7 @@ ssize_t bch2_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	struct bch_inode_info *inode = file_bch_inode(file);
 	struct address_space *mapping = file->f_mapping;
 	size_t count = iov_iter_count(iter);
-	ssize_t ret;
+	ssize_t ret = 0;
 
 	if (!count)
 		return 0; /* skip atime */
@@ -205,7 +205,7 @@ ssize_t bch2_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			iocb->ki_pos += ret;
 	} else {
 		bch2_pagecache_add_get(inode);
-		ret = generic_file_read_iter(iocb, iter);
+		ret = filemap_read(iocb, iter, ret);
 		bch2_pagecache_add_put(inode);
 	}
 out: