fsverity: fix build error by adding fsverity_readahead() stub

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: 45dcb3ac9832 ("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>