dynamic_debug: reduce lineno field to a saner 18 bits

lineno:24 allows files with 4 million lines, an insane file-size, even
for never-to-get-in-tree machine generated code.  Reduce this to 18
bits, which still allows 256k lines.  This is still insanely big, but
its not raving mad.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>