xfs: don't assert fail on bad inode numbers

Let the inode verifier do it's work by returning an error when we
fail to find correct magic numbers in an inode buffer.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>