ext4: handle decryption error in __ext4_block_zero_page_range()

fscrypt_decrypt_pagecache_blocks() can fail, because it uses
skcipher_request_alloc(), which uses kmalloc(), which can fail; and also
because it calls crypto_skcipher_decrypt(), which can fail depending on
the driver that actually implements the crypto.

Therefore it's not appropriate to WARN on decryption error in
__ext4_block_zero_page_range().

Remove the WARN and just handle the error instead.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191226154105.4704-1-ebiggers@kernel.orgSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3725,8 +3725,12 @@ static int __ext4_block_zero_page_range(handle_t *handle,
 		if (S_ISREG(inode->i_mode) && IS_ENCRYPTED(inode)) {
 			/* We expect the key to be set. */
 			BUG_ON(!fscrypt_has_encryption_key(inode));
-			WARN_ON_ONCE(fscrypt_decrypt_pagecache_blocks(
-					page, blocksize, bh_offset(bh)));
+			err = fscrypt_decrypt_pagecache_blocks(page, blocksize,
+							       bh_offset(bh));
+			if (err) {
+				clear_buffer_uptodate(bh);
+				goto unlock;
+			}
 		}
 	}
 	if (ext4_should_journal_data(inode)) {