Fix dump buffers on Sparc

[ghost]

Fix for Linux Sparc where heap addresses have their MSB set, a case that pread() can't handle.

Indeed Linux's pread() implementation has a limitation: it treats the offset argument as a signed offset, hence it can't access the top half of the memory space. And on Sparc, heap addresses have their MSB set, for example:

[~]# cat /proc/self/maps | grep 'rw-p'
7fefff28000-7fefff4a000 rw-p 00000000 00:00 0                            [stack]
fffffc0100000000-fffffc0100002000 rw-p 00000000 00:00 0
fffffc0100004000-fffffc0100006000 rw-p 00000000 00:00 0

So the problem with bootchart-collector using pread() to read from /dev/{pid}/mem becomes obvious: the first call to pread() successfully reads from the stack, but subsequent calls fail reading from the heap and return EINVAL. The fix is to replace the call to pread() by two system calls: lseek() which can handle unsigned offsets and then read(). There is no real performance degradation and it is portable.

[xrmx]

Merged, thanks.