pmem2: ndctl get size

src/libpmem2/pmem2_utils.h

src/libpmem2/pmem2_utils_linux.c

src/libpmem2/pmem2_utils_ndctl.c

src/libpmem2/pmem2_utils_none.c

src/libpmem2/pmem2_utils_other.c

src/libpmem2/source_posix.c