fscrypt: remove redundant assignment of res

res is assigned to sizeof(ctx), however, this is unused and res
is updated later on without that assigned value to res ever being
used.  Remove this redundant assignment.

Fixes CoverityScan CID#1395546 "Unused value"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>