eCryptfs: write lock requested keys

A requested key is write locked in order to prevent modifications on the
authentication token while it is being used.

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

diff --cc fs/ecryptfs/keystore.c
Simple merge

diff --cc fs/ecryptfs/main.c
Simple merge