NFSD: Remove be32_to_cpu() from DRC hash function

Commit 7142b98d

 ("nfsd: Clean up drc cache in preparation for
global spinlock elimination"), billed as a clean-up, added
be32_to_cpu() to the DRC hash function without explanation. That
commit removed two comments that state that byte-swapping in the
hash function is unnecessary without explaining whether there was
a need for that change.

On some Intel CPUs, the swab32 instruction is known to cause a CPU
pipeline stall. be32_to_cpu() does not add extra randomness, since
the hash multiplication is done /before/ shifting to the high-order
bits of the result.

As a micro-optimization, remove the unnecessary transform from the
DRC hash function.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>