net/mlx5: Convert mkey_table to XArray

The lock protecting the data structure does not need to be an rwlock.  The
only read access to the lock is in an error path, and if that's limiting
your scalability, you have bigger performance problems.

Eliminate mlx5_mkey_table in favour of using the xarray directly.
reg_mr_callback must use GFP_ATOMIC for allocating XArray nodes as it may
be called in interrupt context.

This also fixes a minor bug where SRCU locking was being used on the radix
tree read side, when RCU was needed too.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>