LU-13822 ptlrpc: fixes for RCU-related stalls

LU-13822 ptlrpc: fixes for RCU-related stalls

ptlrpc_expired_set() may need to process a lot
of requests, so the processing loop needs to
schedule from time to time to avoid RCU-related
stalls.

Change-Id: I14b0aaf14ab127805699729adbb26459a2f4f07c
Signed-off-by: Andrew Perepechko <andrew.perepechko@hpe.com>
HPE-bug-id: LUS-8939
Reviewed-on: https://review.whamcloud.com/39514
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>