Fix mount failed "Cannot allocate memory".

When the memory gets fragmented, kzalloc() might fail to allocate
physically contiguous pages for the struct exfat_sb_info (its size is
about 34KiB) even the total free memory is enough.
Use kvzalloc() to solve this problem.
Reviewed-by: Ethan Wu <ethanwu@synology.com>
Signed-off-by: Jia-Ye Li <jiayeli@synology.com>
Link: https://lore.kernel.org/r/20190925083729.4653-1-jiayeli@synology.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>