xfs: avoid UAF on sc->tempip in xrep_tempfile_create

LOLLM noticed a potential UAF if the tempfile creation code fails after
it set sc->tempip.  Fix that.

Cc: <stable@vger.kernel.org> # v6.10
Fixes: 84c14ee39dd388 ("xfs: create temporary files and directories for online repair")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>