f2fs: sysfs: add encoding_flags entry

This patch adds a new sysfs entry /sys/fs/f2fs/<disk>/encoding_flags,
it is a read-only entry to show the value of sb.s_encoding_flags, the
value is hexadecimal.

============================     ==========
Flag_Name                        Flag_Value
============================     ==========
SB_ENC_STRICT_MODE_FL            0x00000001
SB_ENC_NO_COMPAT_FALLBACK_FL     0x00000002
============================     ==========

case#1
mkfs.f2fs -f -O casefold -C utf8:strict /dev/vda
mount /dev/vda /mnt/f2fs
cat /sys/fs/f2fs/vda/encoding_flags
1

case#2
mkfs.f2fs -f -O casefold -C utf8 /dev/vda
fsck.f2fs --nolinear-lookup=1 /dev/vda
mount /dev/vda /mnt/f2fs
cat /sys/fs/f2fs/vda/encoding_flags
2

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>