The check for whether to drain retry faults must be under the mmap write
lock to serialize with munmap notifier callbacks.

We were also missing checks on child ranges. To fix that, simplify the
logic by using a flag rather than checking on each prange. That also
allows draining less freqeuntly when many ranges are unmapped at once.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Philip Yang <Philip.Yang@amd.com>
Tested-by: Alex Sierra <Alex.Sierra@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>