f2fs: fix to reset variable correctlly

f2fs_map_blocks will set m_flags and m_len to 0, so we don't need to
reset m_flags ourselves, but have to reset m_len to correct value
before use it again.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>