cfq-iosched: fix cfq_cic_link() race confition

cfq_cic_link() has race condition. When some processes which shared ioc
issue I/O to same block device simultaneously, cfq_cic_link() returns -EEXIST
sometimes. The race condition might stop I/O by following steps:

step  1: Process A: Issue an I/O to /dev/sda
step  2: Process A: Get an ioc (iocA here) in get_io_context() which does not
		    linked with a cic for the device
step  3: Process A: Get a new cic for the device (cicA here) in
		    cfq_alloc_io_context()

step  4: Process B: Issue an I/O to /dev/sda
step  5: Process B: Get iocA in get_io_context() since process A and B share the
		    same ioc
step  6: Process B: Get a new cic for the device (cicB here) in
		    cfq_alloc_io_context() since iocA has not been linked with a
		    cic for the device yet

step  7: Process A: Link cicA to iocA in cfq_cic_link()
step  8: Process A: Dispatch I/O to driver and finish it

step  9: Process B: Try to link cicB to iocA in cfq_cic_link()
		    But it fails with showing "cfq: cic link failed!" kernel
		    message, since iocA has already linked with cicA at step 7.
step 10: Process B: Wait for finishig I/O in get_request_wait()
		    The function does not wake up, when there is no I/O to the
		    device.

When cfq_cic_link() returns -EEXIST, it means ioc has already linked with cic.
So when cfq_cic_link() return -EEXIST, retry cfq_cic_lookup().

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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