lib/vsprintf: Force type of flags value for gfp_t

Sparse is not happy about restricted type being assigned:
  lib/vsprintf.c:1940:23: warning: incorrect type in assignment (different base types)
  lib/vsprintf.c:1940:23:    expected unsigned long [assigned] flags
  lib/vsprintf.c:1940:23:    got restricted gfp_t [usertype]

Force type of flags value to make sparse happy.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20200731180825.30575-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>