rge: make rx_process_limit a sysctl tunable

[chris.longros_gmail.com]

Summary

The number of packets processed per interrupt was hardcoded to 16.
Add a per-interface sysctl dev.rge.%d.rx_process_limit tunable so
users can adjust this value at runtime.

Signed-off-by: Christos Longros <[e-mail]>

[ziaee]

Ok from manpages. No comment on the source change, but I appreciate including robust and concise documentation with the proposed change!