RELENG_7: interrupt eating whole cpu core

[Dominic Fandrey]

While ripping DVDs the irq14: ata0 eats ~95% of one of my cores. The channel 
is exclusive to the drive.

# atacontrol list                                   64 /root
ATA channel 0:
     Master: acd0 <Optiarc DVD RW AD-7560A/DH10> ATA/ATAPI revision 7
     Slave:       no device present
ATA channel 1:
     Master:      no device present
     Slave:       no device present
ATA channel 2:
     Master:  ad4 <ST9160821AS/3.BHE> Serial ATA v1.0
     Slave:       no device present
ATA channel 3:
     Master:      no device present
     Slave:       no device present
ATA channel 4:
     Master:      no device present
     Slave:       no device present


The number of interrupts looks reasonable to me.

# vmstat -i                                          0 /root
interrupt                          total       rate
irq1: atkbd0                       35171          1
irq9: acpi0                        15729          0
irq12: psm0                        10004          0
irq14: ata0                      5148195        272
irq16: pcm0 uhci0                  40783          2
irq17: uhci1+                    1446238         76
irq18: bge0 ehci0+                 38588          2
irq20: uhci2 ehci1                125447          6
irq21: uhci3                      156783          8
cpu0: timer                     37792079       1999
cpu1: timer                     37784062       1999
Total                           82593079       4370


I'm currently rebuilding world+kernel. I will report weather the behaviour has 
changed. This is an HP 6510b GR695EA#ABD, if anyone thinks it might be 
helpful, I can supply you with a dmesg and the output of pciconf -lv.