ext4: always initialize the crc32c checksum driver

The extended attribute code now uses the crc32c checksum for hashing
purposes, so we should just always always initialize it.  We also want
to prevent NULL pointer dereferences if one of the metadata checksum
features is enabled after the file sytsem is originally mounted.

This issue has been assigned CVE-2018-1094.

https://bugzilla.kernel.org/show_bug.cgi?id=199183
https://bugzilla.redhat.com/show_bug.cgi?id=1560788

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
