proc: Helper for opening vma's file

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
proc-parse: Don't open and stat every single map_files link

Quite a lot of VMAs in tasks map the same file with different
perms. In that case we may skip opening all these files, but
"borrow" one from the previous VMA parsed.

There's little sense in seeking more that just previous VMA,
as same files are rarely (can be though) mapped in different
locations.

After this on a basic Centos6 container the number of opens and
stats in this function drops from ~1500 to ~500.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>