PCI/MSI: Warn and return error if driver enables MSI/MSI-X twice

It is a serious driver defect to enable MSI or MSI-X more than once.  Doing
so may panic the kernel as in the stack trace below:

  Call Trace:
    sysfs_add_one+0xa5/0xd0
    create_dir+0x7c/0xe0
    sysfs_create_subdir+0x1c/0x20
    internal_create_group+0x6d/0x290
    sysfs_create_groups+0x4a/0xa0
    populate_msi_sysfs+0x1cd/0x210
    pci_enable_msix+0x31c/0x3e0
    igbuio_pci_open+0x72/0x300 [igb_uio]
    uio_open+0xcc/0x120 [uio]
    chrdev_open+0xa1/0x1e0
    [...]
    do_sys_open+0xf3/0x1f0
    SyS_open+0x1e/0x20
    system_call_fastpath+0x16/0x1b
    ---[ end trace 11042e2848880209 ]---
    Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: ffffffffa056b4fa

We want to keep the WARN_ON() and stack trace so the driver can be fixed,
but we can avoid the kernel panic by returning an error.  We may still get
warnings like this:

  Call Trace:
    pci_enable_msix+0x3c9/0x3e0
    igbuio_pci_open+0x72/0x300 [igb_uio]
    uio_open+0xcc/0x120 [uio]
    chrdev_open+0xa1/0x1e0
    [...]
    do_sys_open+0xf3/0x1f0
    SyS_open+0x1e/0x20
    system_call_fastpath+0x16/0x1b
    ------------[ cut here ]------------
    WARNING: at fs/sysfs/dir.c:526 sysfs_add_one+0xa5/0xd0()
    sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:03.0/0000:01:00.1/msi_irqs'

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
[bhelgaas: changelog, fix patch whitespace, remove !!]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>