Fix missing type in mknodat() mode argument

Fix missing type in mknodat() mode argument

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 set_xattr is used only when extracting regular files, fix that
by ORing its mode argument with S_IFREG.

Copyright-paperwork-exempt: Yes