ANDROID: page_alloc: export symbol for vendor module

Previously, I've added a vendor hook not to use certain type of
pageblocks as migration target with below commit.
https://android-review.googlesource.com/c/kernel/common/+/3475721

For this, export get_pfnblock_flags_mask() function for vendor module
to get migratetype of pageblock.

Bug: 410432482
Change-Id: Ia2d065927ab7288cd7367c41bdd9fb116ba995b0
Signed-off-by: Sooyong Suk <s.suk@samsung.corp-partner.google.com>

mm/page_alloc.c

@@ -450,6 +450,7 @@ unsigned long get_pfnblock_flags_mask(const struct page *page,
 	word = READ_ONCE(bitmap[word_bitidx]);
 	return (word >> bitidx) & mask;
 }
+EXPORT_SYMBOL_GPL(get_pfnblock_flags_mask);
 
 static __always_inline int get_pfnblock_migratetype(const struct page *page,
 					unsigned long pfn)