f2fs, block_dump: give WRITE direction to submit_bio

The block_dump in submit_bio uses rw, instead of bio->bi_rw.

Change-Id: Iacbec6caabff64d188eb1df84f006b41916e1098
Fixes: a3ccc667 ("fscrypt,f2fs: catch up fscrypt and f2fs-stable/v3.18 having v4.12-rc1"
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>