fs: btrfs: Add disk-to-cpu and cpu-to-disk conversion functions

BTRFS on disk structures are stored in Little Endian. Add functions
to convert this structures to cpu and to disk format.

On Little Endian hosts, these functions do nothing.

On Big Endian the CALL_MACRO_FROM_EACH from variadic-macro.h is used
to define all the members for each structure on which cpu_to_le* or
le*_to_cpu is to be called.

Signed-off-by: Marek Behun <marek.behun@nic.cz>

 create mode 100644 fs/btrfs/conv-funcs.h