fscache: Implement a hash function

fscache: Implement a hash function

Implement a function to generate hashes.  It needs to be stable over time
and endianness-independent as the hashes will appear on disk in future
patches.  It can assume that its input is a multiple of four bytes in size
and alignment.

This is borrowed from the VFS and simplified.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com