MFC r259877: Slightly simplify expiration logic introduced in r254337.
MFC r259877:
Slightly simplify expiration logic introduced in r254337.

 - Do not update the histogram for items we are any way deleting from cache.
 - Do not update the histogram if nfsrc_tcphighwater is not set.
 - Remove some extra math operations.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase