LU-12400 ptlrpc: Sun RPC changes for RCU locking

LU-12400 ptlrpc: Sun RPC changes for RCU locking

In kernel 4.20 SUNRPC cache_detail->hash_lock changed to spinlock_t

  Now that the reader functions are all RCU protected, use a regular
  spinlock rather than a reader/writer lock.

Linux-commit: 1863d77f15da0addcd293a1719fa5d3ef8cde3ca

This patch is back-ported from the following one:
Lustre-commit: 77d53777e32c80047cb75293d5f9a4c0d23bbea8
Lustre-change: https://review.whamcloud.com/35499

Test-Parameters: trivial
Cray-bug-id: LUS-7600
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: If0df38337d5a2bb0ac4b8cb645dbe89f65e0f352
Reviewed-on: https://review.whamcloud.com/38483
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>