base_filesystem_create: do not try to create "/root" if it exists

The check, if the directory/file already exists is only executed, if
there is a symlink target specified. In case of "/root", there is none,
so it is unconditionally tried to create the directory.

In case of a readonly filesystem, errno != EEXIST, but errno == EROFS,
so base_filesystem_create() and switch_root does not succeed.

This patch checks for existance not only in the symlink case.