devtmpfs: missing initialialization in never-hit case

create_path() on something without a single / in it will return err
without initializing it.  It actually can't happen (we call that thing
only if create on the same path returns -ENOENT, which won't happen
happen for single-component path), but in this case initializing err
to 0 is more than making compiler to STFU - would be the right thing
to return on such paths; the function creates a parent directory of
given pathname and in that case it has no work to do...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>