[crypto] Use inverse size as effective size for bigint_mod_invert()

[crypto] Use inverse size as effective size for bigint_mod_invert()

Montgomery reduction requires only the least significant element of an
inverse modulo 2^k, which in turn depends upon only the least
significant element of the invertend.

Use the inverse size (rather than the invertend size) as the effective
size for bigint_mod_invert().  This eliminates around 97% of the loop
iterations for a typical 2048-bit RSA modulus.

Signed-off-by: Michael Brown <mcb30@ipxe.org>