modpost: validate symbol names also in find_elf_symbol

If an ARM mapping symbol shares an address with a valid symbol,
find_elf_symbol can currently return the mapping symbol instead, as the
symbol is not validated. This can result in confusing warnings:

  WARNING: vmlinux.o(.text+0x18f4028): Section mismatch in reference
  from the function set_reset_devices() to the variable .init.text:$x.0

This change adds a call to is_valid_name to find_elf_symbol, similarly
to how it's already used in find_elf_symbol2.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>