libnvdimm, btt: fix format string warnings

Fix format warnings (seen on i386) in nvdimm/btt.c:

../drivers/nvdimm/btt.c: In function ‘btt_map_init’:
../drivers/nvdimm/btt.c:430:3: warning: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 4 has type ‘size_t’ [-Wformat=]
   dev_WARN_ONCE(to_dev(arena), size < 512,
   ^
../drivers/nvdimm/btt.c: In function ‘btt_log_init’:
../drivers/nvdimm/btt.c:474:3: warning: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 4 has type ‘size_t’ [-Wformat=]
   dev_WARN_ONCE(to_dev(arena), size < 512,
   ^

Fixes: 86652d2e ("libnvdimm, btt: clean up warning and error messages")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>