f2fs: fix to reset fields for unloaded curseg

In f2fs_allocate_data_block(), before skip allocating new segment
for DATA_PINNED log header, it needs to tag log header as unloaded
one to avoid skipping logic in locate_dirty_segment() and
__f2fs_save_inmem_curseg().

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>