f2fs: preload extent_cache for POSIX_FADV_WILLNEED

This patch tries to preload extent_cache given POSIX_FADV_WILLNEED, which is
more useful for generic usecases.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -4858,6 +4858,9 @@ static int f2fs_file_fadvise(struct file *filp, loff_t offset, loff_t len,
 		filp->f_mode &= ~FMODE_RANDOM;
 		spin_unlock(&filp->f_lock);
 		return 0;
+	} else if (advice == POSIX_FADV_WILLNEED && offset == 0) {
+		/* Load extent cache at the first readahead. */
+		f2fs_precache_extents(inode);
 	}
 
 	err = generic_fadvise(filp, offset, len, advice);