x86/pod: prevent infinite loop when shattering large pages

x86/pod: prevent infinite loop when shattering large pages

When populating pages, the PoD may need to split large ones using
p2m_set_entry and request the caller to retry (see ept_get_entry for
instance).

p2m_set_entry may fail to shatter if it is not possible to allocate
memory for the new page table. However, the error is not propagated
resulting to the callers to retry infinitely the PoD.

Prevent the infinite loop by return false when it is not possible to
shatter the large mapping.

This is XSA-246.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>