The magic dropping of mmap_sem when handle_mm_fault returns VM_FAULT_RETRY
is rather subtile.  Add a comment explaining it.

Link: https://lore.kernel.org/r/20190724065258.16603-8-hch@lst.deTested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
[hch: wrote a changelog]
Signed-off-by: Christoph Hellwig <hch@lst.de>