kstrdup() returns NULL on error.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>