libceph: force GFP_NOIO for socket allocations

sock_alloc_inode() allocates socket+inode and socket_wq with
GFP_KERNEL, which is not allowed on the writeback path:

    Workqueue: ceph-msgr con_work [libceph]
    ffff8810871cb018 0000000000000046 0000000000000000 ffff881085d40000
    0000000000012b00 ffff881025cad428 ffff8810871cbfd8 0000000000012b00
    ffff880102fc1000 ffff881085d40000 ffff8810871cb038 ffff8810871cb148
    Call Trace:
    [<ffffffff816dd629>] schedule+0x29/0x70
    [<ffffffff816e066d>] schedule_timeout+0x1bd/0x200
    [<ffffffff81093ffc>] ? ttwu_do_wakeup+0x2c/0x120
    [<ffffffff81094266>] ? ttwu_do_activate.constprop.135+0x66/0x70
    [<ffffffff816deb5f>] wait_for_completion+0xbf/0x180
    [<ffffffff81097cd0>] ? try_to_wake_up+0x390/0x390
    [<ffffffff81086335>] flush_work+0x165/0x250
    [<ffffffff81082940>] ? worker_detach_from_pool+0xd0/0xd0
    [<ffffffffa03b65b1>] xlog_cil_force_lsn+0x81/0x200 [xfs]
    [<ffffffff816d6b42>] ? __slab_free+0xee/0x234
    [<ffffffffa03b4b1d>] _xfs_...