objtool: Fix off-by-one in symbol_by_offset()

Sometimes, WARN_FUNC() and other users of symbol_by_offset() will
associate the first instruction of a symbol with the symbol preceding
it.  This is because symbol->offset + symbol->len is already outside of
the symbol's range.

Fixes: 2a362ecc ("objtool: Optimize find_symbol_*() and read_symbols()")
Signed-off-by: Julien Thierry <jthierry@redhat.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>