Avoid shadowing in linux-tdep.c

Avoid shadowing in linux-tdep.c

This is one of the uglier changes to avoid local shadowing.  Because
obstack.h uses statement expressions, in some cases a nested obstack
call will result in shadowing.  Rather than try to fix obstack.h, this
patch simply works around the one instance of this problem.

gdb/ChangeLog
2018-10-04  Tom Tromey  <tom@tromey.com>

	* linux-tdep.c (linux_make_mappings_corefile_notes): Introduce new
	variable "size".