linux-user: Allocate the right amount of space for non-fixed file maps

linux-user: Allocate the right amount of space for non-fixed file maps

If we let the kernel handle the implementation of mmap_find_vma,
via an anon mmap, we must use the size as indicated by the user
and not the size truncated to the filesize.

This happens often in ld.so, where we initially mmap the file to
the size of the text+data+bss to reserve an area, then mmap+fixed
over the top to properly handle data and bss.

Signed-off-by: Richard Henderson <rth@twiddle.net>