ubifs: Initialize fstr_real_len

While fstr_real_len is only being used under if (encrypted),
gcc-6 still warns.

Fixes this false positive:
fs/ubifs/dir.c: In function 'ubifs_readdir':
fs/ubifs/dir.c:629:13: warning: 'fstr_real_len' may be used
uninitialized in this function [-Wmaybe-uninitialized]
    fstr.len = fstr_real_len

Initialize fstr_real_len to make gcc happy.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/dir.c

@@ -528,7 +528,7 @@ static unsigned int vfs_dent_type(uint8_t type)
  */
 static int ubifs_readdir(struct file *file, struct dir_context *ctx)
 {
-	int fstr_real_len, err = 0;
+	int fstr_real_len = 0, err = 0;
 	struct fscrypt_name nm;
 	struct fscrypt_str fstr = {0};
 	union ubifs_key key;