sandbox: Correct maths in allocation routines

sandbox: Correct maths in allocation routines

Allocation routines were adjusted to ensure that the returned addresses
are a multiple of the page size, but the header code was not updated to
take account of this. These routines assume that the header size is the
same as the page size which is unlikely.

At present os_realloc() does not work correctly due to this bug. The only
user is the hostfs 'ls' command, and only if the directory contains a
unusually long filename, which likely explains why this bug was not
caught earlier.

Fix this by doing the calculations using the obtained page size.

Signed-off-by: Simon Glass <sjg@chromium.org>