kernel: fix f2fs on big endian machines

kernel: fix f2fs on big endian machines

The WD MyBook Live SquashFS images didn't work anymore due to
a upstream regression in f2fs commit: 0cfe75c5b01199
("f2fs: enhance sanity_check_raw_super() to avoid potential overflows")
that got backported to 4.14.86 and landed in 4.18.

by Martin Blumenstingl:
|Treat "block_count" from struct f2fs_super_block as 64-bit little endian
|value in sanity_check_raw_super() because struct f2fs_super_block
|declares "block_count" as "__le64".
|
|This fixes a bug where the superblock validation fails on big endian
|devices with the following error:
|  F2FS-fs (sda1): Wrong segment_count / block_count (61439 > 0)
|  F2FS-fs (sda1): Can't find valid F2FS filesystem in 1th superblock
|  F2FS-fs (sda1): Wrong segment_count / block_count (61439 > 0)
|  F2FS-fs (sda1): Can't find valid F2FS filesystem in 2th superblock
|As result of this the partition cannot be mounted.
|
|With this patch applied the superblock validation works fine and the
|partition can be mounted again:
|  F2FS-fs (sda1): Mounted with checkpoint version = 7c84
|
|My little endian x86-64 hardware was able to mount the partition without
|this fix.
|To confirm that mounting f2fs filesystems works on big endian machines
|again I tested this on a 32-bit MIPS big endian (lantiq) device.

Hopefully, this will do until Martin's patch moved through upstream
to -stable.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>