Use 64-bit type for packet and content size

Expression filestats.st_size * CHAR_BIT wraps around very easily
when stored to 32-bit size_t. Currently BabelTrace cannot handle
input larger than 256Mb because of this and diagnostic messages
from this function can be incorrect because of overflow.

This patch fixes my immediate problem, but further work is needed
for proper large file support i.e. handling files bigger than 2Gb.

Signed-off-by: Juha Niskanen <juniskane@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>