LU-3581 osc: Lustre returns EINTR from writes when SA_RESTART is set

LU-3581 osc: Lustre returns EINTR from writes when SA_RESTART is set

When Lustre is in a read or write system call and receives a
SIGALRM, it will return EINTR if interrupted in osc_enter_cache.
This prevents the system call from being restarted if
SA_RESTART is set in the handler.

This patch changes behavior in this location to return ERESTARTSYS
when a signal arrives during the call to l_wait_event.

Signed-off-by: Patrick Farrell <paf@cray.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I95423c5d572974352173f3c5c94d980385bfef29
Reviewed-on: http://review.whamcloud.com/7002
Reviewed-by: Rahul Deshmukh <rahul_deshmukh@xyratex.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Cory Spitz <spitzcor@cray.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>