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>