fix mount api

- mount syscall allow `source` to be null

Author: CutieDeng

lib/std/os/linux.zig

@@ -906,7 +906,7 @@ pub fn mknodat(dirfd: i32, path: [*:0]const u8, mode: u32, dev: u32) usize {
     return syscall4(.mknodat, @as(usize, @bitCast(@as(isize, dirfd))), @intFromPtr(path), mode, dev);
 }
 
-pub fn mount(special: [*:0]const u8, dir: [*:0]const u8, fstype: ?[*:0]const u8, flags: u32, data: usize) usize {
+pub fn mount(special: ?[*:0]const u8, dir: [*:0]const u8, fstype: ?[*:0]const u8, flags: u32, data: usize) usize {
     return syscall5(.mount, @intFromPtr(special), @intFromPtr(dir), @intFromPtr(fstype), flags, data);
 }