libxl: dm_restrict: DEFINE_USERLOOKUP_HELPER returned a pointer to an auto

libxl: dm_restrict: DEFINE_USERLOOKUP_HELPER returned a pointer to an auto

When I converted the previous open-coded user lookup functionality
into DEFINE_USERLOOKUP_HELPER, I moved the struct passwd buffer into
the function generated by the macro.  This is wrong because that
buffer is used by get{pw,gr}* for its return value, so the helper
function would contrive to return a pointer to the buffer on its own
stack.

Fix this by adding a buffer parameter to the generated helpers, that
the caller must supply, and updating all the call sites.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>