Btrfs-progs: Fix compiler warnings on PPC64

Btrfs-progs: Fix compiler warnings on PPC64

The kernel uses unsigned long long for u64, but PPC64 uses unsigned
long by default. This results in compilation warnings such as:

print-tree.c:333: warning: format '%llu' expects type 'long long
unsigned int', but argument 4 has type 'u64'

To fix this, the macro __KERNEL__ needs to be defined before including
the file <asm/types.h>. This can be done by defining the macro in
"kerncompat.h" and making it the first included file in the relevant
header files; this fixes the compiler warnings on PPC64.

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Wade Cline <clinew@linux.vnet.ibm.com>