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

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

BugLink: http://bugs.launchpad.net/bugs/1668019
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>
(cherry picked from commit 073ff3c8e6acdd6bae91a037e6f2d0edeed4165d)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>