f2fs: use the updated test_dummy_encryption helper functions

Switch f2fs over to the functions that are replacing
fscrypt_set_test_dummy_encryption().  Since f2fs hasn't been converted
to the new mount API yet, this doesn't really provide a benefit for
f2fs.  But it allows fscrypt_set_test_dummy_encryption() to be removed.

Also take the opportunity to eliminate an #ifdef.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>