dwarf-reader: add support for symbol namespaces in ksymtab entries

dwarf-reader: add support for symbol namespaces in ksymtab entries

Kernel v5.4 introduces Symbol Namespaces [1]. That changes the layout of
ksymtab entries in Kernel binaries. In particular, the kernel_symbol
entry gains a new member to represent the namespace. That change affects
binaries that have position relative relocations (we name that format
here V4_19_KSYMTAB_FORMAT) as well as those that don't
(PRE_V4_19_KSYMTAB_FORMAT). In any case there is an additional entry
that has the same size as the previous entries.

Since we iterate over the ksymtab entries to collect them, we need to
determine the correct size of these entries even though we do not
grab the namespace for ABI analysis purposes at this time.

In order to determine the size, we attempt to find the beginning of the
next entry by trying to read symbols with an increasing offset. Once we
succeed, we have the offset and therefore the size of one entry.

Since try_reading_first_ksymtab_entry() does already everything we need
to attempt to read a symbol from a beginning of a ksymtab, we only
needed to teach it to operate on an offset to read the potential second
entry.

'load_kernel_symbol_table' was determining the number of entries
unconditionally, even when we do have the unsupported case of a ksymtab
with relocations. Hence only load when needed.

	* src/abg-dwarf-reader.cc
	(read_context::try_reading_first_ksymtab_entry): Add
	symbol_offset parameter.
	(read_context::get_ksymtab_entry_size): Add support for variable
	size ksymtab entries due to symbol namespaces.
	(load_kernel_symbol_table): only load nb_entries when needed

[1] https://lore.kernel.org/lkml/20190906103235.197072-1-maennich@google.com/

Signed-off-by: Matthias Maennich <maennich@google.com>