ZFS compiling against Linux git seems to have broken in December

[rincebrain]

System information

Type	Version/Name
Distribution Name	Debian
Distribution Version	10
Linux Kernel	5.12.4
Architecture	x86_64
ZFS Version	7457b02

Describe the problem you're observing

While trying to reproduce #12056 , I ran into this. (See openzfs/zfs-buildbot#227 for my theory as to how this could have happened.)

It appears to have broken with torvalds/linux@4e7b5671c6a88 .

Describe how to reproduce the problem

• Try building with a kernel post that commit.

Include any warning/errors/backtraces from the system logs

  CC      fs/zfs/spl/../os/linux/spl/spl-generic.o
In file included from ./include/zfs/os/linux/spl/sys/uio.h:31,
                 from ./include/zfs/os/linux/spl/sys/sunddi.h:28,
                 from fs/zfs/spl/../os/linux/spl/spl-generic.c:41:
./include/zfs/os/linux/kernel/linux/blkdev_compat.h: In function ‘vdev_lookup_bdev’:
./include/zfs/os/linux/kernel/linux/blkdev_compat.h:328:30: error: too few arguments to function ‘lookup_bdev’
  328 |  struct block_device *bdev = lookup_bdev(path);
      |                              ^~~~~~~~~~~
In file included from ./include/linux/blk-cgroup.h:23,
                 from ./include/linux/writeback.h:14,
                 from ./include/linux/memcontrol.h:22,
                 from ./include/linux/swap.h:9,
                 from ./include/zfs/os/linux/spl/sys/vmsystm.h:29,
                 from fs/zfs/spl/../os/linux/spl/spl-generic.c:28:
./include/linux/blkdev.h:1984:5: note: declared here
 1984 | int lookup_bdev(const char *pathname, dev_t *dev);
      |     ^~~~~~~~~~~
make[3]: *** [scripts/Makefile.build:271: fs/zfs/spl/../os/linux/spl/spl-generic.o] Error 1
make[2]: *** [scripts/Makefile.build:514: fs/zfs/spl] Error 2
make[1]: *** [scripts/Makefile.build:514: fs/zfs] Error 2
make: *** [Makefile:1851: fs] Error 2

edit:
Fascinatingly, somehow this doesn't get spit out when I use bb-build-linux.sh instead of cp /boot/config-$(uname -r) .config;make olddefconfig; make -j12;

So that's odd.

[rincebrain]

Nevermind, I wasn't passing the right flags to ./configure, so it was picking fun from 5.10.X and then copy-builtin over to 5.12.X.