f2fs crypto: add missing locking for keyring_key access

commit 745e8490b1e960ad79859dd8ba6a0b5a8d3d994e upstream.

This patch adopts:
	ext4 crypto: add missing locking for keyring_key access

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>