Add a new stat that counts mmap failures, which might help when debugging
different issues.

Link: https://lore.kernel.org/r/20200420062213.44577-3-galpress@amazon.comReviewed-by: Firas JahJah <firasj@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>