loopback_test: reorder and fix argument validation
loopback_test: reorder and fix argument validation

This patch makes two related changes:
1. move validation of -t and -i after -l processing, so that
   "loopback_test -l" (list loopback devices and exit) will
   work.
2. fix validation for test name present by testing name[0] rather than
   &name.  Now, loopback_test will detect when the mandatory
   "-t <test name>" is missing.

Signed-off-by: Mitchell Tasman <195b19bf4fecfcb68944a885eddeb58a2d122c5a@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@google.com>

projectara/gbsim,gregkh/gbsim,projectara/gbsim,gregkh/gbsim