Do cleanup more for better code readability.

- Change the parameter set of f2fs_bio_alloc()
  This function should allocate a bio only since it is not something like
  f2fs_bio_init(). Instead, the caller should initialize the allocated bio.

- Introduce SECTOR_FROM_BLOCK
  This macro translates a block address to its sector address.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>