Changes

add jhash.h, copied from linux/jhash.h

Allow using arbitrary length data as hashtable key:
Copy jhash.h implementation from the linux kernel.
Apply osmo_ prefix to all global symbols.

Add jhash_test to ensure the code import works as intended.

First application will be a hashtable keyed by umts_cell_id in
osmo-hnbgw.git.

Related: SYS#6773
Change-Id: I0c9652bbc9e2a18b1200e7d63bb6f64ded7d75fa