ANDROID: sync generic casefolding code with patches going upstream

Get the generic casefolding code in sync with the patches that are
queued in f2fs.git#dev for 5.10.

Equivalently, this reverts the patch
"ANDROID-fs-adjust-casefolding-support-to-match-android-mainline.patch"
from the android-mainline quilt series, with the following conflicts:

Conflicts:
	fs/ext4/hash.c  # due to "ANDROID: ext4: Handle casefolding with encryption"
	fs/ext4/namei.c # due to "ANDROID: ext4: Handle casefolding with encryption"
	fs/f2fs/dir.c   # due to "ANDROID: f2fs: Handle casefolding with Encryption"

Bug: 161184936
Cc: Daniel Rosenberg <drosen@google.com>
Cc: Paul Lawrence <paullawrence@google.com>
Cc: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I0ae169f0f5f413fb21e4be7a163213aef3fa6756
Signed-off-by: Eric Biggers <ebiggers@google.com>