LU-13587 quota: protect qpi in proc

LU-13587 quota: protect qpi in proc

Access to pool info only when pool is fully inited.
This patch protects from following panic:

[212010.467347] BUG: unable to handle kernel NULL pointer dereference at           (null)
[212010.468205] IP: [<ffffffffc0e55e46>] qpi_state_seq_show+0x86/0xe0 [lquota]
...
[212010.486786] Call Trace:
[212010.487344]  [<ffffffffbbc68b50>] seq_read+0x130/0x440
[212010.487741]  [<ffffffffbbcb8380>] proc_reg_read+0x40/0x80
[212010.488445]  [<ffffffffbbc4118f>] vfs_read+0x9f/0x170
[212010.489056]  [<ffffffffbbc4204f>] SyS_read+0x7f/0xf0
[212010.489920]  [<ffffffffbc176ddb>] system_call_fastpath+0x22/0x27
[212010.490861] Code: 5c a8 01 00 00 41 8b 8c 1c c0 01 00 00 48 c7 c6 18
[212010.493235] RIP  [<ffffffffc0e55e46>] qpi_state_seq_show+0x86/0xe0 [lquota]
[212010.493672] RSP <ffff908505747e28>
[212010.494161] CR2: 0000000000000000

Add test 79 to sanity-quota to check that race between
access to /proc/.../dt-pool_name/info of non-existed pool
with this pool creating doesn't cause a panic.

HPE-bug-id: LUS-9938
Change-Id: I8eff846c6c3881a8431a98efb54e660ecb9155bf
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/43987
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>