fscrypto: make filename crypto functions return 0 on success

Several filename crypto functions: fname_decrypt(),
fscrypt_fname_disk_to_usr(), and fscrypt_fname_usr_to_disk(), returned
the output length on success or -errno on failure.  However, the output
length was redundant with the value written to 'oname->len'.  It is also
potentially error-prone to make callers have to check for '< 0' instead
of '!= 0'.

Therefore, make these functions return 0 instead of a length, and make
the callers who cared about the return value being a length use
'oname->len' instead.  For consistency also make other callers check for
a nonzero result rather than a negative result.

This change also fixes the inconsistency of fname_encrypt() actually
already returning 0 on success, not a length like the other filename
crypto functions and as documented in its function comment.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 34c0142..2243ae2 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -639,7 +639,7 @@
 					res = fscrypt_fname_alloc_buffer(
 						dir, len,
 						&fname_crypto_str);
-					if (res < 0)
+					if (res)
 						printk(KERN_WARNING "Error "
 							"allocating crypto "
 							"buffer--skipping "
@@ -647,7 +647,7 @@
 					res = fscrypt_fname_disk_to_usr(dir,
 						0, 0, &de_name,
 						&fname_crypto_str);
-					if (res < 0) {
+					if (res) {
 						printk(KERN_WARNING "Error "
 							"converting filename "
 							"from disk to usr"
@@ -1011,7 +1011,7 @@
 			err = fscrypt_fname_disk_to_usr(dir, hinfo->hash,
 					hinfo->minor_hash, &de_name,
 					&fname_crypto_str);
-			if (err < 0) {
+			if (err) {
 				count = err;
 				goto errout;
 			}
@@ -3144,7 +3144,7 @@
 		istr.name = (const unsigned char *) symname;
 		istr.len = len;
 		err = fscrypt_fname_usr_to_disk(inode, &istr, &ostr);
-		if (err < 0)
+		if (err)
 			goto err_drop_inode;
 		sd->len = cpu_to_le16(ostr.len);
 		disk_link.name = (char *) sd;