bnxt(4) Enable LRO support, redux

iflib - reset fl-ifl_fragidx to 0 on iflib_fl_bufs_free(). This caused the
panic in em/igb when adding it to a bridge device.

iflib - Handle out of order packet delivery from hardware in support of LRO

Out of order updates to rxd's is fixed in r315217. However, it is not
completely fixed. While refilling the buffers, iflib is not considering
the out of order descriptors. Hence, it is refilling sequentially.
"idx" variable in _iflib_fl_refill routine is incremented sequentially.
By doing refilling sequentially, it will override the SGEs that
are *IN USE* by other connections. Fix is to maintain a bitmap of
rx descriptors and differentiate the used one with unused one and
refill only at the unused indices. This patch also fixes a
few bugs in bnxt, related to the same feature.

Submitted by: bhargava.marreddy@broadcom.com
Reviewed by: venkatkumar.duvvuru@broadcom.com shurd
Differential Revision: https://reviews.freebsd.org/D10681