fscrypt: properly declare on-stack completion

When a completion is declared on-stack we have to use
COMPLETION_INITIALIZER_ONSTACK().

Fixes: 0b81d077 ("fs crypto: move per-file encryption from f2fs
tree to fs/crypto")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>