Add the 'whint_mode' mount option that controls which write
hints are passed down to block layer. There are "off" and
"user-based" mode. The default mode is "off".

1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.

2) whint_mode=user-based. F2FS tries to pass down hints given
by users.

User                  F2FS                     Block
----                  ----                     -----
                      META                     WRITE_LIFE_NOT_SET
                      HOT_NODE                 "
                      WARM_NODE                "
                      COLD_NODE                "
ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
extension list        "                        "

-- buffered io
WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
WRITE_LIFE_NONE       "                        "
WRITE_LIFE_MEDIUM     "                        "
WRITE_LIFE_LONG       "                        "

-- direct io
WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
WRITE_LIFE_NONE       "                        WRITE_LIFE_NONE
WRITE_LIFE_MEDIUM     "                        WRITE_LIFE_MEDIUM
WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG

Many thanks to Chao Yu and Jaegeuk Kim for comments to
implement this patch.
Signed-off-by: Hyunchul Lee <cheol.lee@lge.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: avoid build warning]
[Chao Yu: fix to restore whint_mode in ->remount_fs]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>