The Xarray allows us to store the cached mkeys in memory efficient way.

Entries are reserved in the Xarray using xa_cmpxchg before calling to the
upcoming callbacks to avoid allocations in interrupt context.  The
xa_cmpxchg can sleep when using GFP_KERNEL, so we call it in a loop to
ensure one reserved entry for each process trying to reserve.

Link: https://lore.kernel.org/r/20220726071911.122765-3-michaelgur@nvidia.comSigned-off-by: NAharon Landau <aharonl@nvidia.com>
Signed-off-by: NLeon Romanovsky <leonro@nvidia.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>