xfs_io: modify argument errors of mremap command

xfs_io: modify argument errors of mremap command

There are two argument parsing errors in mremap command:

1. "mremap 1024 8192" won't return error and will run "mremap 1024"
silently.

2. The "-f" option can't be used, due to it needing a new address
argument as the fifth argument of mremap() syscall.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>