mbuf: fix free space update for dynamic field

mbuf: fix free space update for dynamic field

The value free_space[i] is used to save the size of biggest aligned
element that can fit in the zone, current implementation has one flaw,
for example, if user registers dynfield1 (size = 4, align = 4, req = 124)
first, the free_space would be as below after registration:

  0070: 08 08 08 08 08 08 08 08
  0078: 08 08 08 08 00 00 00 00

Then if user continues to register dynfield2 (size = 4, align = 4),
free_space would become:

  0070: 00 00 00 00 04 04 04 04
  0078: 04 04 04 04 00 00 00 00

Further request dynfield3 (size = 8, align = 8) would fail to register
due to alignment requirement can't be satisfied, though there is enough
space remained in mbuf.

This patch fixes above issue by saving alignment only in aligned zone,
after the fix, above registrations order can be satisfied, free_space
would be like:

After dynfield1 registration:

  0070: 08 08 08 08 08 08 08 08
  0078: 04 04 04 04 00 00 00 00

After dynfield2 registration:

  0070: 08 08 08 08 08 08 08 08
  0078: 00 00 00 00 00 00 00 00

After dynfield3 registration:

  0070: 00 00 00 00 00 00 00 00
  0078: 00 00 00 00 00 00 00 00

This patch also reduces iterations in process_score() by jumping align
steps in each loop.

Fixes: 4958ca3a443a ("mbuf: support dynamic fields and flags")
Cc: stable@dpdk.org
Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>