MFC r338489:

MFC r338489:
Maximum number of mbuf frags is off-by-one for worst case scenario in mlx5en(4).

Inspecting the PRM no more than 0x3F data segments, DS, of size 16 bytes is
allowed.

Worst case scenario summary of DS usage:
Header is fixed:	2 DS
Maximum inlining:	98 => (98 - 2) / 16 = 6 DS
Remainder:		0x3F - 2 - 6 = 55 DS (mbuf frags)

Previously a value of 56 DS was used and this would work in the
normal case because not all inline data area was used up.

Sponsored by:		Mellanox Technologies

git-svn-id: svn://svn.freebsd.org/base/stable/10@338549 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f