UBI: Fastmap: Fix kernel-doc

drivers/mtd/ubi/fastmap.c:104: warning: expecting prototype for new_fm_vhdr(). Prototype was for new_fm_vbuf() instead.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2289


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>