linux-user/elfload: fix address calculation in fallback scenario

linux-user/elfload: fix address calculation in fallback scenario

Previously, guest_loaddr was not taken into account when returning an
address from pgb_find_hole when /proc/self/maps was unavailable which
caused an improper guest_base address to be calculated.

This could cause a SIGSEGV later in load_elf_image -> target_mmap for
ET_EXEC type images since the mmap MAP_FIXED flag is specified which
could clobber existing mappings at the address returnd by g2h().

  mmap(0xd87000, 16846912, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|0x100000, -1, 0) = 0xd87000
  munmap(0xd87000, 16846912)              = 0
  write(2, "Locating guest address space @ 0"..., 40Locating guest address space @ 0xd87000) = 40
  mmap(0x1187000, 16850944, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x1187000
  --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_ACCERR, si_addr=0x2188310} ---
  +++ killed by SIGSEGV +++

Now, pgd_find_hole accounts for guest_loaddr in this scenario.

Fixes: ad592e37dfcc ("linux-user: provide fallback pgd_find_hole for bare chroots")
Signed-off-by: Vincent Fazio <vfazio@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210131061948.15990-1-vfazio@xes-inc.com>
[lv: updated it to check if ret == -1]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>