elf-loader: Fix truncation warning from coverity

elf-loader: Fix truncation warning from coverity

Coverity reports a truncation due to cast operation on operand
reltab->sh_size from 64 bits to 32 bits for calls of load_at.

Fix the types of the function arguments to match their use in
function load_at: the offset is used for lseek which takes an
off_t parameter, the size is used for g_malloc and read.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>