f2fs crypto: replace some BUG_ON()'s with error checks

commit 66aa3e1274fcf887e9d6501a68163270fc7718e7 upstream.

This patch adopts:
	ext4 crypto: replace some BUG_ON()'s with error checks

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>