ubifs: free the encrypted symlink target

ubifs_symlink() forgot to free the kmalloc()'ed buffer holding the
encrypted symlink target, creating a memory leak.  Fix it.

(UBIFS could actually encrypt directly into ui->data, removing the
temporary buffer, but that is left for the patch that switches to use
the symlink helper functions.)

Fixes: ca7f85be

 ("ubifs: Add support for encrypted symlinks")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>