xenpaging: handle GNTST_eagain in kernel drivers

xenpaging: handle GNTST_eagain in kernel drivers

Handle GNTST_eagain status from GNTTABOP_map_grant_ref and
GNTTABOP_copy operations properly to allow usage of xenpaging without
causing crashes or data corruption.

Remove the existing retry code from net_rx_action(),
dispatch_rw_block_io(), net_accel_map_grants_contig() and
net_accel_map_iomem_page() and replace all relevant
HYPERVISOR_grant_table_op() calls with a retry loop.  This loop is
implemented as a macro to allow different GNTTABOP_* args.  It will
sleep up to 33 seconds and wait for the page to be paged in again.

All ->status checks were updated to use the GNTST_* namespace.  All
return values are converted from GNTST_* namespace to 0/-EINVAL, since
all callers did not use the actual return value.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Patrick Colp <pjcolp@cs.ubc.ca>