Use KM_PUSHPAGE in taskq_dispatch_delay()

[behlendorf]

Running 0.6.2 on Ubuntu 12.04 LTS with 3.8.0-30 kernel:
[40557.683345] SPL: Fixing allocation for task txg_sync (616) which used GFP flags 0xfb9e9ba4 with PF_NOFS set
[40557.683351] SPL: Showing stack for process 616
[40557.683355] Pid: 616, comm: txg_sync Tainted: PF O 3.8.0-30-generic #44~precise1-Ubuntu
[40557.683358] Call Trace:
[40557.683381] [<ffffffffa00724d7>] spl_debug_dumpstack+0x27/0x40 [spl]
[40557.683393] [<ffffffffa00853e4>] sanitize_flags.part.10+0x68/0xc84 [spl]
[40557.683400] [<ffffffffa00774d3>] kmem_alloc_debug+0x303/0x3b0 [spl]
[40557.683404] [<ffffffff8135bbe2>] ? put_dec+0x72/0x90
[40557.683407] [<ffffffff8135cb05>] ? number.isra.2+0x355/0x390
[40557.683414] [<ffffffffa007725e>] ? kmem_alloc_debug+0x8e/0x3b0 [spl]
[40557.683421] [<ffffffffa007b370>] task_alloc+0x1a0/0x350 [spl]
[40557.683427] [<ffffffffa007725e>] ? kmem_alloc_debug+0x8e/0x3b0 [spl]
[40557.683463] [<ffffffffa0184f20>] ? spa_add+0x4f0/0x4f0 [zfs]
[40557.683470] [<ffffffffa007bbeb>] taskq_dispatch_delay+0x19b/0x2b0 [spl]
[40557.683478] [<ffffffffa007d1ab>] ? taskq_cancel_id+0xeb/0x1e0 [spl]
[40557.683481] [<ffffffff810ae88c>] ? ktime_get_ts+0x4c/0xe0
[40557.683507] [<ffffffffa0177a82>] spa_sync+0x1f2/0xae0 [zfs]
[40557.683510] [<ffffffff810ae88c>] ? ktime_get_ts+0x4c/0xe0
[40557.683537] [<ffffffffa0188cef>] txg_sync_thread+0x2df/0x540 [zfs]
[40557.683564] [<ffffffffa0188a10>] ? txg_init+0x250/0x250 [zfs]
[40557.683572] [<ffffffffa007ac08>] thread_generic_wrapper+0x78/0x90 [spl]
[40557.683579] [<ffffffffa007ab90>] ? __thread_create+0x310/0x310 [spl]
[40557.683582] [<ffffffff8107f1b0>] kthread+0xc0/0xd0
[40557.683585] [<ffffffff8107f0f0>] ? flush_kthread_worker+0xb0/0xb0
[40557.683589] [<ffffffff816fcaec>] ret_from_fork+0x7c/0xb0
[40557.683591] [<ffffffff8107f0f0>] ? flush_kthread_worker+0xb0/0xb0

The call trace appears a few dozen more times in the syslog, 5 seconds apart. The pool was under load from about a dozen concurrent rsync jobs.

[behlendorf]

Fixed by cbfa294 which has already been merged.