BACKPORT: filemap: add a comment about FAULT_FLAG_RETRY_NOWAIT behavior

I thought Josef Bacik's patch to drop the mmap_sem was buggy, because
when looking at the error cases, there was one case where we returned
VM_FAULT_RETRY without actually dropping the mmap_sem.

Josef had to explain to me (using small words) that yes, that's actually
what we're supposed to do, and his patch was correct.  Which not only
convinced me he knew what he was doing and I should stop arguing with
him, but also that I should add a comment to the case I was confused
about.

Bug: 124328118
Patiently-pointed-out-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Minchan Kim <minchan@google.com>
(cherry picked from commit 8b0f9fa2e02dc95216577c3387b0707c5f60fbaf)
Change-Id: I2971fd1c817958748bcfb10938bd145467e7448e