RDMA/uverbs: Use uverbs_request() and core for write_ex handlers

The write_ex handlers have this horrible boilerplate in every function to
do the zero extend/zero check and min size checks. This is now handled in
the core code via the meta-data, and the zero checks are handled by
uverbs_request(). Replace all the occurrences.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>