ext4: lock the xattr block before checksuming it

We must lock the xattr block before calculating or verifying the
checksum in order to avoid spurious checksum failures.

https://bugzilla.kernel.org/show_bug.cgi?id=193661

Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org