xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests

stable inclusion
from stable-v5.10.152
commit 5232411f37d79654db574ff7357c5acc4620f173
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73HJ0

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5232411f37d79654db574ff7357c5acc4620f173



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[ Upstream commit 30dcc56b ]

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: 5c13a4a0 ("xen/gntdev: Accommodate VMA splitting")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lipeng Sang <sanglipeng1@jd.com>