f2fs: compress: remove unneeded f2fs_put_dnode()

If we don't initialize dn.inode_page for f2fs_get_block(),
f2fs_get_block() will call f2fs_put_dnode() itself, so let's
remove unneeded f2fs_put_dnode() in f2fs_vm_page_mkwrite().

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>