net: ipv4: hashtables: avoid division by zero compilation error

Since sizeof(spinlock_t) is calculated everytime and may be used as
divisor in equation, compiler throws an error division by zero. Use
a variable to store the value of sizeof(spinlock_t) and in equations.

Change-Id: I60908c71ca1fe373211f7955e75a1a5889dcf8e2
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>