stat-util: struct stat could be initialized with (st_mode & S_IFMT == 0)

stat-util: struct stat could be initialized with (st_mode & S_IFMT == 0)

For anonymous inodes, the result would be 0, but
the struct stat is initialized obviously.
So let's switch to st_dev for the check, which
is guaranteed to be non-zero.

Also this is completely unnecessary for statx(),
since we check stx_mask first and that on its own
denotes that the struct is initialized.