mm: add prototype for __add_to_page_cache_locked()

Otherwise it causes a gcc warning:

  mm/filemap.c:830:14: warning: no previous prototype for `__add_to_page_cache_locked' [-Wmissing-prototypes]

A previous attempt to make this function static led to compilation
errors when CONFIG_DEBUG_INFO_BTF is enabled because
__add_to_page_cache_locked() is referred to by BPF code.

Adding a prototype will silence the warning.

Link: https://lkml.kernel.org/r/1608693702-4665-1-git-send-email-jrdr.linux@gmail.comSigned-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -216,6 +216,13 @@ int overcommit_kbytes_handler(struct ctl_table *, int, void *, size_t *,
 		loff_t *);
 int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
 		loff_t *);
+/*
+ * Any attempt to mark this function as static leads to build failure
+ * when CONFIG_DEBUG_INFO_BTF is enabled because __add_to_page_cache_locked()
+ * is referred to by BPF code. This must be visible for error injection.
+ */
+int __add_to_page_cache_locked(struct page *page, struct address_space *mapping,
+		pgoff_t index, gfp_t gfp, void **shadowp);
 
 #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))