Per POSIX, the type of the file to be created should be OR'ed to the
`mode` argument of mknodat().

However, set_xattr() creates an empty file using mknodat() and does not
do that.

E.g. Linux kernel considers zero type as S_IFREG, so the code works on
most systems.

However, e.g. fakeroot, at least up to the current v1.36, does not
consider 0 as S_IFREG, instead creating an invalid file, causing tar to
enter an infinite retry loop when trying to create a file with xattrs
under fakeroot.

Since the current code is incorrect per POSIX, simply fix that by
calling mknodat() with S_IFREG.

This will cause no behavior difference on systems where 0 already was
considered S_IFREG.