f2fs: introduce f2fs_update_dentry to clean up duplicated codes

This patch introduces f2fs_update_dentry to remove redundant code in
f2fs_add_inline_entry and __f2fs_add_link.

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