ubifs: Fix kernel-doc

Fix function name in fs/ubifs/io.c kernel-doc comment
to remove some warnings found by clang(make W=1 LLVM=1).

fs/ubifs/io.c:497: warning: expecting prototype for
wbuf_timer_callback(). Prototype was for wbuf_timer_callback_nolock()
instead
fs/ubifs/io.c:513: warning: expecting prototype for new_wbuf_timer().
Prototype was for new_wbuf_timer_nolock() instead
fs/ubifs/io.c:538: warning: expecting prototype for cancel_wbuf_timer().
Prototype was for cancel_wbuf_timer_nolock() instead

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>