selinux: improve bucket distribution uniformity of avc_hash()

selinux: improve bucket distribution uniformity of avc_hash()

Reuse the already implemented MurmurHash3 algorithm. Under heavy stress
testing (on an 8-core system sustaining over 50,000 authentication events
per second), sample once per second and take the mean of 1800 samples:

1. Bucket utilization rate and length of longest chain
+--------------------------+-----------------------------------------+
|                          | bucket utilization rate / longest chain |
|                          +--------------------+--------------------+
|                          |      no-patch      |     with-patch     |
+--------------------------+--------------------+--------------------+
|  512 nodes,  512 buckets |      52.5%/7.5     |     60.2%/5.7      |
+--------------------------+--------------------+--------------------+
| 1024 nodes,  512 buckets |      68.9%/12.1    |     80.2%/9.7      |
+--------------------------+--------------------+--------------------+
| 2048 nodes,  512 buckets |      83.7%/19.4    |     93.4%/16.3     |
+--------------------------+--------------------+--------------------+
| 8192 nodes, 8192 buckets |      49.5%/11.4    |     60.3%/7.4      |
+--------------------------+--------------------+--------------------+

2. avc_search_node latency (total latency of hash operation and table
lookup)
+--------------------------+-----------------------------------------+
|                          |   latency of function avc_search_node   |
|                          +--------------------+--------------------+
|                          |      no-patch      |     with-patch     |
+--------------------------+--------------------+--------------------+
|  512 nodes,  512 buckets |        87ns        |        84ns        |
+--------------------------+--------------------+--------------------+
| 1024 nodes,  512 buckets |        97ns        |        96ns        |
+--------------------------+--------------------+--------------------+
| 2048 nodes,  512 buckets |       118ns        |       113ns        |
+--------------------------+--------------------+--------------------+
| 8192 nodes, 8192 buckets |       106ns        |        99ns        |
+--------------------------+--------------------+--------------------+

Although MurmurHash3 has higher overhead than the bitwise operations in
the original algorithm, the data shows that the MurmurHash3 achieves
better distribution, reducing average lookup time. Consequently, the
total latency of hashing and table lookup is lower than before.

Signed-off-by: Hongru Zhang <zhanghongru@xiaomi.com>
[PM: whitespace fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>