FROMLIST: fscrypt: Have filesystems handle their d_ops

This shifts the responsibility of setting up dentry operations from
fscrypt to the individual filesystems, allowing them to have their own
operations while still setting fscrypt's d_revalidate as appropriate.

Also added helper function to libfs to unify ext4 and f2fs
implementations.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Test: Boots, /data/media is case insensitive
Bug: 138322712
Link: https://lore.kernel.org/linux-f2fs-devel/20200208013552.241832-1-drosen@google.com/T/#t
Change-Id: Iaf77f8c5961ecf22e22478701ab0b7fe2025225d