generic/446: make sure all background processes are dead before umount

generic/446: make sure all background processes are dead before umount

The $dread_pid refers to the while-true-do loop, wait for $dread_pid
doesn't mean the xfs_io direct read process is already dead,
sometimes xfs_io process is still running and blocking
_scratch_unmount.

Fix it by making the direct read does a fixed number of loop and
break out the second mmap-fpunch loop if the first loop exits. At
this point we're sure that there's no unfinished background process
blocking the umount.

Reviewed-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>