FROMLIST: Add standard casefolding support

This adds general supporting functions for filesystems that use
utf8 casefolding. It provides standard dentry_operations and adds the
necessary structures in struct super_block to allow this standardization.

Ext4 and F2fs are switch to these implementations.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Note: Fixed issue with non-strictly enforced fallback hash
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: I81b5fb5d3ce0259a60712ae2505c1e4b03dbafde