[PATCH v2] nvmet: protect sqhd update by a lock

[Johannes Thumshirn]

Still somewhat awkward to read, maybe we can fold in a comment on why the lock
only touches these two lines and isn't used outside of this function anymore.

Anyways,
Reviewed-by: Johannes Thumshirn <jthumshirn at suse.de>

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Johannes Thumshirn                                          Storage
jthumshirn at suse.de                                +49 911 74053 689
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