f2fs: call bdev_zone_sectors() only once on init_blkz_info()

Instead of calling bdev_zone_sectors() multiple times, call
it once and cache the value locally. This will make the
subsequent change easier to read.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>