server: only do cpu_to_le64() on io_sample_data member if iolog is histogram

server: only do cpu_to_le64() on io_sample_data member if iolog is histogram

In the case of histogram iolog, the union io_sample_data member is a
pointer of struct io_u_plat_entry, while in the case of normal iolog, it
is an uint64_t. Thus only need to do the byteswap in case it is an
uint64_t.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Signed-off-by: Tuan Hoang <tuan.hoang1@ibm.com>