ext4: reject casefold inode flag without casefold feature

ext4: reject casefold inode flag without casefold feature

It is invalid for the casefold inode flag to be set without the casefold
superblock feature flag also being set.  e2fsck already considers this
case to be invalid and handles it by offering to clear the casefold flag
on the inode.  __ext4_iget() also already considered this to be invalid,
sort of, but it only got so far as logging an error message; it didn't
actually reject the inode.  Make it reject the inode so that other code
doesn't have to handle this case.  This matches what f2fs does.

Note: we could check 's_encoding != NULL' instead of
ext4_has_feature_casefold().  This would make the check robust against
the casefold feature being enabled by userspace writing to the page
cache of the mounted block device.  However, it's unsolvable in general
for filesystems to be robust against concurrent writes to the page cache
of the mounted block device.  Though this very particular scenario
involving the casefold feature is solvable, we should not pretend that
we can support this model, so let's just check the casefold feature.
tune2fs already forbids enabling casefold on a mounted filesystem.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230814182903.37267-2-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>