linux-user: test, don't assert addr != test in pgb_reserved_va

linux-user: test, don't assert addr != test in pgb_reserved_va

On older kernels which don't implement MAP_FIXED_NOREPLACE the kernel
may still fail to give us the address we asked for despite having
already probed the map for a valid hole. Asserting isn't particularly
useful to the user so let us move the check up and expand the
error_report a little to give them a fighting chance of working around
the problem.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Bug 1895080 <1895080@bugs.launchpad.net>
Ameliorates: ee94743034
Message-Id: <20200915134317.11110-2-alex.bennee@linaro.org>