ext4: fix error message when rejecting the default hash

Commit 985b67cd ("ext4: filesystems without casefold feature cannot
be mounted with siphash") properly rejects volumes where
s_def_hash_version is set to DX_HASH_SIPHASH, but the check and the
error message should not look into casefold setup - a filesystem should
never have DX_HASH_SIPHASH as the default hash.  Fix it and, since we
are there, move the check to ext4_hash_info_init.

Fixes:985b67cd ("ext4: filesystems without casefold feature cannot
be mounted with siphash")

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://patch.msgid.link/87jzg1en6j.fsf_-_@mailhost.krisman.be


Signed-off-by: Theodore Ts'o <tytso@mit.edu>