s3:smbd: Fix converity warning with _smb_setlen_large()

s3:smbd: Fix converity warning with _smb_setlen_large()

result_independent_of_operands: "(outsize - 4 & 0xffffff) >> 16 >> 8" is
0 regardless of the values of its operands. This occurs as the bitwise
first operand of "&".

So we should just pass a variable to silence the warning. However for
this, we should calculate it correctly and use size_t for it.

Found by Coverity.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>