net/mlx5: Use exact encap header size for the FW input buffer

The current code is allocating the max encap size supported by
the firmware and not the size requested by the caller, fix that.

Also, spare a warning when the size of the encapsulation headers
is bigger from what is supported by the firmware.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>