xen-netback: fix LOR

lock order reversal: (sleepable after non-sleepable)
1st 0xfffffe00357ff538 xnb_softc (xen netback softc lock) @ /usr/src/sys/dev/xen/netback/netback.c:1069
2nd 0xffffffff81fdccb0 intrsrc (intrsrc) @ /usr/src/sys/x86/x86/intr_machdep.c:224

There's no need to hold the lock since the cleaning of the interrupt
cannot happen in parallel due to the XNBF_IN_SHUTDOWN flag being set.
Note that the locking in netback needs some improvement or
clarification.

While there also remove a double newline.

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