xfs: check that br_blockcount doesn't overflow

xfs_bmbt_irec.br_blockcount is declared as xfs_filblks_t, which is an
unsigned 64-bit integer.  Though the bmbt helpers will never set a value
larger than 2^21 (since the underlying on-disk extent record has a
length field that is only 21 bits wide), we should be a little defensive
about checking that a bmbt record doesn't exceed what we're expecting or
overflow into the next AG.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>