RDMA/mlx5: remove redundant check on err on return expression

RDMA/mlx5: remove redundant check on err on return expression

Currently all paths that set err and then check it for an error
perform immediate returns, hence err always zero at the end of
the function _mlx5r_umr_zap_mkey.  The return expression
err ? err : nblocks has a redundant check on the err since err
is always zero, so just return nblocks instead.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://patch.msgid.link/20250717112108.4036171-1-colin.i.king@gmail.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>