f2fs: make f2fs_sync_inode_meta() static

After commit 26b5a079 ("f2fs: cleanup dirty pages if recover failed"),
f2fs_sync_inode_meta() is only used in checkpoint.c, so
f2fs_sync_inode_meta() should only be visible inside. Delete the
declaration in the header file and change f2fs_sync_inode_meta()
to static.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>