fsverity: fix build error by adding fsverity_readahead() stub

hppa-linux-gcc 9.5.0 generates a call to fsverity_readahead() in
f2fs_readahead() when CONFIG_FS_VERITY=n, because it fails to do the
expected dead code elimination based on vi always being NULL.  Fix the
build error by adding an inline stub for fsverity_readahead().  Since
it's just for opportunistic readahead, just make it a no-op.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602180838.pwICdY2r-lkp@intel.com/
Fixes: 45dcb3a ("f2fs: consolidate fsverity_info lookup")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20260218012244.18536-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: Eric Biggers

include/linux/fsverity.h

@@ -195,6 +195,8 @@ int fsverity_ioctl_read_metadata(struct file *filp, const void __user *uarg);
 
 /* verify.c */
 
+void fsverity_readahead(struct fsverity_info *vi, pgoff_t index,
+			unsigned long nr_pages);
 bool fsverity_verify_blocks(struct fsverity_info *vi, struct folio *folio,
 			    size_t len, size_t offset);
 void fsverity_verify_bio(struct fsverity_info *vi, struct bio *bio);
@@ -255,6 +257,11 @@ static inline int fsverity_ioctl_read_metadata(struct file *filp,
 
 /* verify.c */
 
+static inline void fsverity_readahead(struct fsverity_info *vi, pgoff_t index,
+				      unsigned long nr_pages)
+{
+}
+
 static inline bool fsverity_verify_blocks(struct fsverity_info *vi,
 					  struct folio *folio, size_t len,
 					  size_t offset)
@@ -303,8 +310,6 @@ static inline int fsverity_file_open(struct inode *inode, struct file *filp)
 }
 
 void fsverity_cleanup_inode(struct inode *inode);
-void fsverity_readahead(struct fsverity_info *vi, pgoff_t index,
-			unsigned long nr_pages);
 
 struct page *generic_read_merkle_tree_page(struct inode *inode, pgoff_t index);
 void generic_readahead_merkle_tree(struct inode *inode, pgoff_t index,