[libata] use kmap_atomic(KM_IRQ0) in SCSI simulator

We are inside spin_lock_irqsave().  quoth akpm's debug facility:

 [  231.948000] SCSI device sda: 195371568 512-byte hdwr sectors (100030 MB)
 [  232.232000] ata1.00: configured for UDMA/33
 [  232.404000] WARNING (1) at arch/i386/mm/highmem.c:47 kmap_atomic()
 [  232.404000]  [<c01162e6>] kmap_atomic+0xa9/0x1ab
 [  232.404000]  [<c0242c81>] ata_scsi_rbuf_get+0x1c/0x30
 [  232.404000]  [<c0242caf>] ata_scsi_rbuf_fill+0x1a/0x87
 [  232.404000]  [<c0243ab2>] ata_scsiop_mode_sense+0x0/0x309
 [  232.404000]  [<c01729d5>] end_bio_bh_io_sync+0x0/0x37
 [  232.404000]  [<c02311c6>] scsi_done+0x0/0x16
 [  232.404000]  [<c02311c6>] scsi_done+0x0/0x16
 [  232.404000]  [<c0242dcc>] ata_scsi_simulate+0xb0/0x13f
[...]

Signed-off-by: Jeff Garzik <jeff@garzik.org>