IB/uverbs: Hold the uobj write lock after allocate

This clarifies the design intention that time between allocate and
commit has the uobj exclusive to the caller. We already guarantee
this by delaying publishing the uobj pointer via idr_insert,
fd_install, list_add, etc.

Additionally holding the usecnt lock during this period provides
extra clarity and more protection against future mistakes.

Fixes: 38321256 ("IB/core: Add support for idr types")
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>