x86/EPT: correct double unmap_domain_page() on error path

x86/EPT: correct double unmap_domain_page() on error path

c/s 3d90d6e6 "x86/EPT: split super pages upon mismatching memory types"
accidentally introduced an error path where the epte domain page would be
unmapped twice if splitting the superpage failed.

Only unmap the page if the loop is to be continued.  When breaking from the
loop, the page will be unmapped by the subsequent code.

Coverity-ID: 1203047
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>