lmb: Return -EEXIST in lmb_add_region_flags() if region already added

lmb: Return -EEXIST in lmb_add_region_flags() if region already added

An attempt to add the already added LMB region using
lmb_add_region_flags() ends up in lmb_addrs_overlap() check, which
eventually leads to either returning 0 if 'flags' is LMB_NONE, or -1
otherwise. It makes it impossible for the user of this function to catch
the case when the region is already added and differentiate it from
regular errors. That in turn may lead to incorrect error handling in the
caller code, like reporting misleading errors or interrupting the normal
code path where it could be treated as the normal case. An example is
boot_fdt_reserve_region() function, which might be called twice (e.g.
during board startup in initr_lmb(), and then during 'booti' command
booting the OS), thus trying to reserve exactly the same memory regions
described in the device tree twice, which produces an error message on
second call.

Return -EEXIST error code in case when the added region exists and it's
not LMB_NONE; for LMB_NONE return 0, to conform to unit tests
(specifically test_alloc_addr() in test/lib/lmb.c) and the preferred
behavior described in commit 1d9aa4a283da ("lmb: Fix the allocation of
overlapping memory areas with !LMB_NONE"). The change of
lmb_add_region_flags() return values is described in the table below:

    Return case                        Pre-1d9   1d9    New
    -----------------------------------------------------------
    Added successfully                    0      0      0
    Failed to add                         -1     -1     -1
    Already added, flags == LMB_NONE      0      0      0
    Already added, flags != LMB_NONE      0      -1     -EEXIST

Rework all affected functions and their documentation. Also fix the
corresponding unit test which checks reserving the same region with the
same flags to account for the changed return value.

No functional change is intended (by this patch itself).

Fixes: 1d9aa4a283da ("lmb: Fix the allocation of overlapping memory areas with !LMB_NONE")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>