modpost: file2alias: define size of alias

The size of the variable alias provided to do_entry functions are
currently not readily available. Thus hindering do_entry functions to
perform bounds checking.

Define the macro ALIAS_SIZE containing the size of the variable alias.

Signed-off-by: Mattias Jacobsson <2pi@mok.nu>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>