When number of tasks assigned to the specified cpuset becomes zero and
if we try to read tasks file, We get this warning on 2.6.22-rc3 kernel.

=======================================================
------------[ cut here ]------------
Badness at mm/slab.c:777
Call Trace:
[c000000071ebb4a0] [c00000000000eb30] .show_stack+0x68/0x1b0 (unreliable)
[c000000071ebb540] [c00000000021d7b0] .report_bug+0x94/0xe8
[c000000071ebb5d0] [c00000000048eb4c] __kprobes_text_start+0x164/0x5a0
[c000000071ebb650] [c000000000004a84] program_check_common+0x104/0x180
--- Exception: 700 at .__kmalloc+0x44/0x178
    LR = .cpuset_tasks_open+0x90/0x1e8
[c000000071ebb940] [c000000071ebb9e0] 0xc000000071ebb9e0 (unreliable)
[c000000071ebb9e0] [c0000000000812b0] .cpuset_tasks_open+0x90/0x1e8
[c000000071ebbaa0] [c0000000000811fc] .cpuset_file_open+0x70/0x94
[c000000071ebbb30] [c0000000000c2578] .__dentry_open+0x13c/0x278
[c000000071ebbbe0] [c0000000000c2828] .do_filp_open+0x50/0x70
[c000000071ebbd00] [c0000000000c28bc] .do_sys_open+0x74/0x130
[c000000071ebbdb0] [c000000000101108] .compat_sys_open+0x24/0x38
[c000000071ebbe30] [c00000000000862c] syscall_exit+0x0/0x40
=====================================================

This is because "npids"(represnets number of pids in that cpuset) in
"cpu_task_open" is zero and it tries allocate 0 bytes through kmalloc.

Below patch fixes this problem. Please let me know your comments on this.

Signed-off-by: Srinivasa DS <srinivasa@in.ibm.com>