util: Introduce ovs_scan_len()

util: Introduce ovs_scan_len()

This is similar to ovs_scan but takes int pointer as extra
parameter, this pointer point to starting index of the string.
On successful scan this API stores number of characters
scanned.  This API is useful for parsing complex odp actions
e.g. tun_push action.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Ben Pfaff <blp@nicira.com>