Use FIELD_SIZEOF macro instead of hard coding it in field_avail macro.

Link: https://lore.kernel.org/r/20190826115350.21718-3-galpress@amazon.comReviewed-by: Daniel Kranzdorf <dkkranzd@amazon.com>
Reviewed-by: Firas JahJah <firasj@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>