f2fs: remove else in f2fs_write_cache_pages()

As Christoph Hellwig point out:

	Please avoid the else by doing the goto in the branch.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>