RDMA/uverbs: Use UVERBS_ATTR_MIN_SIZE correctly and uniformly

This newer macro allows specifying a lower bound on the accepted size, and
has an 'unlimited' upper bound. Due to this it never checks for trailing
zeroing so it doesn't make any sense to combine it with MIN_SZ_OR_ZERO, so
drop MIN_SZ_OR_ZERO when they are used together

There were a couple of places that open coded this pattern, switch them to
use the clearer UVERBS_ATTR_MIN_SIZE for clarity.

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