efivarfs: Defer PM notifier registration until .fill_super

syzbot reports an issue that turns out to be caused by the fact that the
efivarfs PM notifier may be invoked before the efivarfs_fs_info::sb
field is populated, resulting in a NULL deference.

So defer the registration until efivarfs_fill_super() is invoked.

Reported-by:  <syzbot+00d13e505ef530a45100@syzkaller.appspotmail.com>
Tested-by:  <syzbot+00d13e505ef530a45100@syzkaller.appspotmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>