netfilter: ipset: Fix overflow before widen in the bitmap_ip_create() function.

In the Linux kernel, the following vulnerability has been resolved:

netfilter: ipset: Fix overflow before widen in the bitmap_ip_create() function.

When first_ip is 0, last_ip is 0xFFFFFFFF, and netmask is 31, the value of

an arithmetic expression 2 << (netmask - mask_bits - 1) is subject

to overflow due to a failure casting operands to a larger data type

before performing the arithmetic.

Note that it's harmless since the value will be checked at the next step.

Found by InfoTeCS on behalf of Linux Verification Center

(linuxtesting.org) with SVACE.