selinux: improve role transition hashing

The number of buckets is calculated by performing a binary AND against
the mask of the hash table, which is one less than its size (which is a
power of two).  This leads to all top bits being discarded, e.g. with
the Reference Policy on Debian there exists 376 entries, leading to a
size of 512, discarding the top 23 bits.

Use jhash to improve the hash table utilization:

    # current
    roletr:  376 entries and 124/512 buckets used,
             longest chain length 8, sum of chain length^2 1496

    # patch
    roletr:  376 entries and 266/512 buckets used,
             longest chain length 4, sum of chain length^2 646

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
[PM: line wrap in the commit description]
Signed-off-by: Paul Moore <paul@paul-moore.com>