xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests

xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests

[ Upstream commit 30dcc56bba911db561c35d4131baf983a41023f8 ]

XENFEAT_gnttab_map_avail_bits is always set in Xen 4.0 and newer.
Remove coding assuming it might be zero.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210730071804.4302-4-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Stable-dep-of: 5c13a4a0291b ("xen/gntdev: Accommodate VMA splitting")
Signed-off-by: Sasha Levin <sashal@kernel.org>