After commit c89128a0 ("ext4: handle errors on
ext4_commit_super"), 'ret' may be set to 0 before calling
ext4_fill_flex_info(), if ext4_fill_flex_info() fails ext4_mount()
doesn't return error code, it makes 'root' is null which causes crash
in legacy_get_tree().

Fixes: c89128a0 ("ext4: handle errors on ext4_commit_super")
Reported-by: NHulk Robot <hulkci@huawei.com>
Cc: <stable@vger.kernel.org> # v4.18+
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210510111051.55650-1-yangyingliang@huawei.comSigned-off-by: NTheodore Ts'o <tytso@mit.edu>