libfs: Add generic function for setting dentry_ops

This adds a function to set dentry operations at lookup time that will
work for both encrypted filenames and casefolded filenames.

A filesystem that supports both features simultaneously can use this
function during lookup preparations to set up its dentry operations once
fscrypt no longer does that itself.

Currently the casefolding dentry operation are always set if the
filesystem defines an encoding because the features is toggleable on
empty directories. Unlike in the encryption case, the dentry operations
used come from the parent. Since we don't know what set of functions
we'll eventually need, and cannot change them later, we enable the
casefolding operations if the filesystem supports them at all.

By splitting out the various cases, we support as few dentry operations
as we can get away with, maximizing compatibility with overlayfs, which
will not function if a filesystem supports certain dentry_operations.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>