IB/uverbs: Add helper to get array size from ptr attribute

When the parser of an ioctl command has the knowledge that a ptr attribute
in a bundle represents an array of structures, it is useful for it to know
the number of elements in the array. This is done by dividing the
attribute length with the element size.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Reviewed-by: Guy Levi <guyle@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>