mlxsw: spectrum: Unify delay handling between PFC and pause

When a priority is marked as lossless using DCB PFC, or when pause frames
are enabled on a port, mlxsw adds to port buffers an extra space to cover
the traffic that will arrive between the time that a pause or PFC frame is
emitted, and the time traffic actually stops. This is called the delay. The
concept is the same in PFC and pause, however the way the extra buffer
space is calculated differs.

In this patch, unify this handling. Delay is to be measured in bytes of
extra space, and will not include MTU. PFC handler sets the delay directly
from the parameter it gets through the DCB interface.

To convert pause handler, move MLXSW_SP_PAUSE_DELAY to ethtool module,
convert to bytes, and reduce it by maximum MTU, and divide by two. Then it
has the same meaning as the delay_bytes set by the PFC handler.

Keep the delay_bytes value in struct mlxsw_sp_hdroom introduced in the
previous patch. Change PFC and pause handlers to store the new delay value
there and have __mlxsw_sp_port_headroom_set() take it from there.

Instead of mlxsw_sp_pfc_delay_get() and mlxsw_sp_pg_buf_delay_get(),
introduce mlxsw_sp_hdroom_buf_delay_get() to calculate the delay provision.
Drop the unnecessary MLXSW_SP_CELL_FACTOR, and instead add an explanatory
comment describing the formula used.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>