vma: Don't lookup file_desc for vma twice

We do it first -- on collect, second -- on restore. The
2nd lookup is excessive, we can put fd pointer on vm_area
at lookup and reuse one later.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fifo: Don't lookup reg path twice

Same for previous patch with vmas -- we do it on collect and
on real open. Just put the pointer on fifo_info structure.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
files-reg: Sanitize open_xxx routines a bit

After previous fixes we can merge and move a couple
of them for easier reading.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
files: Add comment to open_path routine

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