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.  le32_to_cpu() is added to
make it endianness-independent.

Changes
=======
ver #3:
 - Read the data being hashed in an endianness-independent way[1].
 - Change the size parameter to be in bytes rather than words.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@mail.gmail.com [1]
Link: https://lore.kernel.org/r/163819586113.215744.1699465806130102367.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906888735.143852.10944614318596881429.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967082342.1823006.8915671045444488742.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021493624.640689.9990442668811178628.stgit@warthog.procyon.org.uk/ # v4