ecryptfs: Change return type of ecryptfs_process_flags

Change return type of ecryptfs_process_flags from int to void as it
never fails.

fixes below issue reported by coccicheck

s/ecryptfs/crypto.c:870:5-7: Unneeded variable: "rc". Return "0" on line
883

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
[tyhicks: Remove the return value line from the function documentation]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>