strstrip(): mark as as must_check

strstrip() can return a modified value of its input argument, when
removing elading whitesapce.  So it is surely bug for this function's
return value to be ignored.  The caller is probably going to use the
incorrect original pointer.

So mark it __must_check to prevent this frm happening (as it has before).

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>