rx: Get rid of AFS_GLOBAL_RXLOCK_KERNEL

rx: Get rid of AFS_GLOBAL_RXLOCK_KERNEL

Get rid of the AFS_GLOBAL_RXLOCK_KERNEL #define. RX used to have a
single global lock locking mode, but none of our kernel modules use
it any more. In fact, AFS_GLOBAL_RXLOCK_KERNEL is now only defined
when RX_ENABLE_LOCKS is also defined. Simplify the code by renaming
all of the occurrences of AFS_GLOBAL_RXLOCK_KERNEL as RX_ENABLE_LOCKS,
and remove any cases where we're now doing unecessary tests

Change-Id: I061d8d364c61168370b223cdac48ad161f0c9d74
Reviewed-on: http://gerrit.openafs.org/8292
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>