xfs_repair: further improvement on secondary superblock search method

xfs_repair: further improvement on secondary superblock search method

This patch is a further optimization of secondary sb search, in
order to handle non-default geometries. Once again, use a similar
method to find fs geometry as that of xfs_mkfs. Refactor
verify_sb(), creating new sub-function that checks sanity of
agblocks and agcount: verify_sb_blocksize().

If verify_sb_blocksize verifies sane paramters, use found values for
the sb search. Otherwise, try search with default values. If these
faster methods both fail, fall back to original brute force slower
search.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>