f2fs: support aligned pinned file

This patch supports 2MB-aligned pinned file, which can guarantee no GC at all
by allocating fully valid 2MB segment.

Check free segments by has_not_enough_free_secs() with large budget.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -154,6 +154,8 @@ static ssize_t features_show(struct f2fs_attr *a,
 	if (f2fs_sb_has_casefold(sbi))
 		len += snprintf(buf + len, PAGE_SIZE - len, "%s%s",
 				len ? ", " : "", "casefold");
+	len += snprintf(buf + len, PAGE_SIZE - len, "%s%s",
+				len ? ", " : "", "pin_file");
 	len += snprintf(buf + len, PAGE_SIZE - len, "\n");
 	return len;
 }