f2fs-tools: use pointer and memory alloaction instead of defining "structure stat...

f2fs-tools: use pointer and memory alloaction instead of defining "structure stat" in function stack

Problem:
Function f2fs_dev_is_unmounted() and get_device_info() define local
variable "struct stat xxx". If the callstack is very deep and stack
is smaller, it will result in stack corruption.

Solution:
It is better to use pointer and memory allocation instead of defining
"structure stat" in function stack.

Signed-off-by: Iris Chang <iris.chang@mediatek.com>
[Jaegeuk Kim: fix build errors.]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>