luci-base: tweak busybox top regex to accept 'm' in large VSZ

Tweak the lua regex string for process list, so that processes
with extremely large memory sizes (over 100 MB) are matched.
For those large processes busybox top shows a MB value like '234m'
instead of a normal kB value like 234000.

Fixes #4425

Reference to https://github.com/openwrt/luci/issues/4425#issuecomment-694073479

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>