checksyscalls: Fix stand-alone usage

The usage help in the comments
  - refers to the wrong script name,
  - doesn't mention that $srctree must be set.

Hence correct the script name, and derive the source tree path from the script
path, so we no longer need to rely on $srctree being set by the caller.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>