LU-11542 import: Fix missing spin_unlock()

LU-11542 import: Fix missing spin_unlock()

A recent patch moved the spin_unlock() down into
each branch of an 'if', but missed the final 'else'.
Add the spin_unlock in the else.

Fixes: 29904135df67 ("LU-11542 import: fix race between imp_state & imp_invalid")
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I6ee399050aad0fe9df9c0e3ddf8ec0be8eae1641
Reviewed-on: https://review.whamcloud.com/35999
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>