Return error code in case of invalid command line option

Currently kexec returns success even if an invalid command line option
is encountered. Kexec currently prints usage message and then returns
0. That's not right. It is an error and error code 1 should be returned.

Due to this wrapper script thinks that kdump succeeded but that's not
the case.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>