xfs: xfs_sb_read_verify() doesn't flag bad crcs on primary sb

My earlier commit 10e6e65d deserves a layer or two of brown paper
bags.  The logic in that commit means that a CRC failure on the
primary superblock will *never* result in an error return.

Hopefully this fixes it, so that we always return the error
if it's a primary superblock, otherwise only if the filesystem
has CRCs enabled.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>