x86: fix typo in recent find_vma_prev purge

It turns out that test-compiling this file on x86-64 doesn't really
help, because much of it is x86-32-specific.  And so I hadn't noticed
the slightly over-eager removal of the 'r' from 'addr' variable despite
thinking I had tested it.

Signed-off-by: Linus "oopsie" Torvalds <torvalds@linux-foundation.org>