ecryptfs: remove redundant variable found_auth_tok

The found_auth_tok variable is no longer needed, as the fact of finding
a token is determined directly by jumping to the found_matching_auth_tok
label inside the loop.

Remove found_auth_tok, simplifying the function logic.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Alexey Velichayshiy <a.velichayshiy@ispras.ru>
[tyhicks: Unsplit log message string]
Signed-off-by: Tyler Hicks <code@tyhicks.com>