NetLabel: Cleanup the LSM domain hash functions

The NetLabel/LSM domain hash table search function used an argument to specify
if the default entry should be returned if an exact match couldn't be found in
the hash table.  This is a bit against the kernel's style so make two separate
functions to represent the separate behaviors.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>