From Anton Altaparmakov

The below bk patch against your current bk repository adds an export
for fs/buffer.c::unmap_underlying_metadata() to modules.

It is required to make ntfs compile as a module in the current kernel.

Note that NTFS doesn't currently dirty bufferheads of the underlying
blockdevice and as such could probably live without calling
unmap_underlying_metadata() but as soon as we enable writing to
compressed files it will do so and considering some of the
infrastructure code is already present I would rather not rip it out
now.