Add two new functions to the AGP driver KPI to bind/unbind arbitrary sets
of pages into the GTT.

Reviewed by:	kib
MFC after:	1 month
Move ifq drain into in6m_purge().

Suggested by:	bms
MFC after:	1 week
Sponsored by:	Yandex LLC
Remove redundant code.
if_detach already did these steps. Also, now we didn't keep routes to link-local
addresses.

Obtained from:	Yandex LLC
Sponsored by:	Yandex LLC
Remove two functions unused after r273848.  Would be nice if clang
or at least scan-build yelled about it.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Port the TTM AGP backend to the FreeBSD agp driver and enable AGP support
in the radeonkms driver.

Note: In PCI mode virtual addresses on the graphics card that map to system
RAM are translated to physical addresses by the graphics card itself.   In
AGP mode address translation is done by the AGP chipset so fictitious
addresses appear on the system bus.  For the CPU cache management to work
correctly when the CPU accesses this memory it needs to use the same
fictitious addresses (and let the chipset translate them) instead of using
the physical addresses directly.

Reviewed by:	kib
MFC after:	1 month
Avoid possible overflow in agp_generic_alloc_memory.

MFC after:	1 week
Make UEFI booting of 4Kn disks work:
      -	convert boot1.efi to corrrectly calculate the lba for what the
	media reports and convert the size based on what FreeBSD uses.
	The existing code would use the 512 byte lba and convert the
	size using 4K byte size.
      -	make fsck_msdosfs read the boot block as 4K so the read doesn't
	fail on a 4Kn drive since FreeBSD will error out parition reads
	of a block.  Make the bpbBytesPerSec check a multiple of 512 since
	it can be 512 or 4K depending on the disk.  This allows fsck to
	pass checking the EFI partition on a 4Kn disk.

To create the EFI file system I used:
	newfs_msdos -F 32 -S 4096 -c 1 -m 0xf8 <partition>
This works for booting 512 and 4Kn disks.

Caveat is that loader.efi cannot read the 4Kn EFI partition.  This isn't
critical right now since boot1.efi will read loader.efi from the ufs
partition.  It looks like loader.efi can be fixed via making some of the
512 bytes reads more flexible.  loader.efi doesn't have trouble reading
the ufs partition.  This is probably a simple fix.

I now have FreeBSD installed on a system with 4Kn drives and tested the
same code works on 512.

MFC after:	1 week
Plug memory ordering holes in buf_ring_enqueue. For at least some
users this patch eliminates the races previously discussed on the
mailing list.

Submitted by:	oleg
Reviewed by:	kmacy
MFC after:	2 weeks
Tested by:	kmacy,rpaulo
Fix compile warning by removing unused variable.

MFC:		3 days
Sponsored by:	Mellanox Technologies