Comment 1 for bug 1465136

A patch in newer kernel (3.19 and later) has a fix for this as below:

commit 2484f40780b97df1b5eb09e78ce4efaa78b21875
Author: Dan McLeran <email address hidden>
Date: Tue Jul 1 09:33:32 2014 -0600

    NVMe: Add shutdown timeout as module parameter.

    The current implementation hard-codes the shutdown timeout to 2 seconds.
    Some devices take longer than this to complete a normal shutdown.
    Changing the shutdown timeout to a module parameter with a default
    timeout of 5 seconds.

    Signed-off-by: Dan McLeran <email address hidden>
    Signed-off-by: Matthew Wilcox <email address hidden>
    Signed-off-by: Jens Axboe <email address hidden>

However, this patch is not included in 3.13 or 3.16