x86/p2m-pt: fix (latent) page table mapping leak on do_recalc() error paths

x86/p2m-pt: fix (latent) page table mapping leak on do_recalc() error paths

There are two mappings active in the middle of do_recalc(), and hence
commit 0d0f4d78e5d1 ("p2m: change write_p2m_entry to return an error
code") should have added (or otherwise invoked) unmapping code just
like it did in p2m_next_level(), despite us not expecting any errors
here. Arrange for the existing unmap invocation to take effect in all
cases.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>