This lock only serializes ucontext creation. Instead of checking the
ucontext_lock during destruction hold the existing hw_destroy_rwsem during
creation, which is the standard pattern for object creation.

The simplification of locking is needed for the next patch.

Link: https://lore.kernel.org/r/1578506740-22188-4-git-send-email-yishaih@mellanox.comSigned-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>