xen/flask: change bool_maxstr to PAGE_SIZE

xen/flask: change bool_maxstr to PAGE_SIZE

When FLASK_{GET,SET}BOOL is called with a named boolean, the call to
flask_security_resolve_bool is made prior to bool_maxstr being populated
by flask_security_make_bools.  This results in the maximum string length
being specified as zero, which is not useful.  While it would be
possible to initialize bool_maxstr correctly prior to its use, it is
simpler to use a fixed maximum of PAGE_SIZE as is done for the other
calls to safe_copy_string_from_guest.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>