linux-user: don't use MAP_FIXED in pgd_find_hole_fallback

linux-user: don't use MAP_FIXED in pgd_find_hole_fallback

Plain MAP_FIXED has the undesirable behaviour of splatting exiting
maps so we don't actually achieve what we want when looking for gaps.
We should be using MAP_FIXED_NOREPLACE. As this isn't always available
we need to potentially check the returned address to see if the kernel
gave us what we asked for.

Fixes: ad592e37dfc ("linux-user: provide fallback pgd_find_hole for bare chroots")
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-Id: <20200724064509[email protected]>