LU-4423 ptlrpc: simplify locking in ptlrpc_add_rqs_to_pool()

LU-4423 ptlrpc: simplify locking in ptlrpc_add_rqs_to_pool()

This spinlock is only needed in this function to
protect the list_add_tail(), so make that more
obvious.

This change means that prp_rq_size is set outside of the spinlock, but
as it is only ever read outside of the spinlock, that cannot matter.

Change-Id: Ie986bf3737f0da157d991d8880148512bc4dd777
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35511
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>