mem: Use long for memory size

When checkpointing applications with really big
memory slab (like in our vz7 test with 920G of
memory) the int type get cutted, we should use
long int instead, just like we do in other code
pieces.

Otherwise get (on vz7's criu, which s sharing the code)

 | pie: 756: Daemon waits for command
 | (01.193097) Wait for ack 12 on daemon socket
 | (01.193112) Fetched ack: 12 12 0
 | (01.193164) 988065 fdinfo 0: pos:                0 flags:           100002/0
 | (01.193201) fdinfo: type: 0xb flags: 0100002/0 pos:        0 fd: 0
 | (01.193279) 988065 fdinfo 1: pos:                0 flags:           100002/0
 | (01.193307) fdinfo: type: 0xb flags: 0100002/0 pos:        0 fd: 1
 | (01.193341) 988065 fdinfo 2: pos:                0 flags:           100002/0
 | (01.193365) fdinfo: type: 0xb flags: 0100002/0 pos:        0 fd: 2
 | (01.193375) ----------------------------------------
 | (01.193405) Error (criu/parasite-syscall.c:243): BUG at criu/parasite-syscall.c:243
 | pie: 756: Error (criu/pie/parasite.c:676): Trimmed message received (1>

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>