mm: add casts to/from gfp_t in gfp_to_alloc_flags()

This removes following warning from sparse:

 mm/page_alloc.c:1934:9: warning: restricted gfp_t degrades to integer

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>