proc: remove m_cache_vma

Instead of setting m->version in the show method, set it in m_next(),
where it should be.  Also remove the fallback code for failing to find a
vma, or version being zero.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200317193201.9924-2-adobriyan@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>