LU-10776 osc: Do not request more than 2GiB grant

LU-10776 osc: Do not request more than 2GiB grant

The server enforces a grant limit of 2 GiB, which the
client must honor.  The existing client code combined with
16 MiB RPCs make it possible for the client to ask for
more than this limit.

Make this limit explicit, and also fix an overflow bug in
o_undirty calculation in osc_announce_cached.  (o_undirty
is a 32 bit value and 16 MiB*256 rpcs_in_flight = 4 GiB.
4 GiB + extra grant components overflows o_undirty.)

Cray-bug-id: LUS-5750
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ifcb8a9ea7529eae4cd209dc72223ed039c6f6a0d
Reviewed-on: https://review.whamcloud.com/31533
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>