fscrypt: handle blocksize < PAGE_SIZE in fscrypt_zeroout_range()

Adjust fscrypt_zeroout_range() to encrypt a block at a time rather than
a page at a time, so that it works when blocksize < PAGE_SIZE.

This isn't optimized for performance, but then again this function
already wasn't optimized for performance.  As a future optimization, we
could submit much larger bios here.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.
Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/bio.c

@@ -72,12 +72,12 @@ EXPORT_SYMBOL(fscrypt_enqueue_decrypt_bio);
 int fscrypt_zeroout_range(const struct inode *inode, pgoff_t lblk,
 				sector_t pblk, unsigned int len)
 {
+	const unsigned int blockbits = inode->i_blkbits;
+	const unsigned int blocksize = 1 << blockbits;
 	struct page *ciphertext_page;
 	struct bio *bio;
 	int ret, err = 0;
 
-	BUG_ON(inode->i_sb->s_blocksize != PAGE_SIZE);
-
 	ciphertext_page = fscrypt_alloc_bounce_page(GFP_NOWAIT);
 	if (!ciphertext_page)
 		return -ENOMEM;
@@ -85,7 +85,7 @@ int fscrypt_zeroout_range(const struct inode *inode, pgoff_t lblk,
 	while (len--) {
 		err = fscrypt_crypt_block(inode, FS_ENCRYPT, lblk,
 					  ZERO_PAGE(0), ciphertext_page,
-					  PAGE_SIZE, 0, GFP_NOFS);
+					  blocksize, 0, GFP_NOFS);
 		if (err)
 			goto errout;
 
@@ -95,14 +95,11 @@ int fscrypt_zeroout_range(const struct inode *inode, pgoff_t lblk,
 			goto errout;
 		}
 		bio_set_dev(bio, inode->i_sb->s_bdev);
-		bio->bi_iter.bi_sector =
-			pblk << (inode->i_sb->s_blocksize_bits - 9);
+		bio->bi_iter.bi_sector = pblk << (blockbits - 9);
 		bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
-		ret = bio_add_page(bio, ciphertext_page,
-					inode->i_sb->s_blocksize, 0);
-		if (ret != inode->i_sb->s_blocksize) {
+		ret = bio_add_page(bio, ciphertext_page, blocksize, 0);
+		if (WARN_ON(ret != blocksize)) {
 			/* should never happen! */
-			WARN_ON(1);
 			bio_put(bio);
 			err = -EIO;
 			goto errout;