Add missing function parameter descriptions:

../drivers/infiniband/core/verbs.c:257: warning: Function parameter or member 'flags' not described in '__ib_alloc_pd'
../drivers/infiniband/core/verbs.c:257: warning: Function parameter or member 'caller' not described in '__ib_alloc_pd'

Link: https://lore.kernel.org/r/20191010035240.011497492@gmail.comSigned-off-by: Randy Dunlap <rd.dunlab@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>