coredump: don't bother with do_truncate()

have dump_skip() just remember how much needs to be skipped,
leave actual seeks/writing zeroes to the next dump_emit()
or the end of coredump output, whichever comes first.
And instead of playing with do_truncate() in the end, just
write one NUL at the end of the last gap (if any).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/binfmts.h

@@ -86,6 +86,7 @@ struct coredump_params {
 	unsigned long mm_flags;
 	loff_t written;
 	loff_t pos;
+	loff_t to_skip;
 };
 
 /*