checkpatch: change externals to globals

Make error message say 'ERROR: do not initialise globals to 0 or NULL'
rather than 'ERROR: do not initialise externals to 0 or NULL'.  Makes more
sense in the context since there is an extern keyword in C and that is a
global declaration within the scope of the current file.

Signed-off-by: Joe Eloff <kagen101@gmail.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>