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>

scripts/checksyscalls.sh

@@ -6,7 +6,7 @@
 # and listed below so they are ignored.
 #
 # Usage:
-# syscallchk gcc gcc-options
+# checksyscalls.sh gcc gcc-options
 #
 
 ignore_list() {
@@ -204,5 +204,5 @@ sed -n -e '/^\#define/ s/[^_]*__NR_\([^[:space:]]*\).*/\
 \#endif/p' $1
 }
 
-(ignore_list && syscall_list ${srctree}/arch/x86/include/asm/unistd_32.h) | \
+(ignore_list && syscall_list $(dirname $0)/../arch/x86/include/asm/unistd_32.h) | \
 $* -E -x c - > /dev/null