f2fs: inode: fix to do sanity check on extent cache correctly

commit 269d1194 upstream.

In do_read_inode(), sanity check for extent cache should be called after
f2fs_init_read_extent_tree(), fix it.

Fixes: 72840ccc

 ("f2fs: allocate the extent_cache by default")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>