f2fs: fix to do sanity check on extent cache correctly

In do_read_inode(), sanity_check_inode() 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>