ixl(4): Report RX errors as sum of all RX error counters

ixl(4): Report RX errors as sum of all RX error counters

HW keeps track of RX errors using several counters, each for
specific type of errors. Report RX errors to OS as sum
of all those counters: CRC errors, illegal bytes, checksum,
length, undersize, fragment, oversize and jabber errors.

There is no HW counter for frames with invalid L3/L4 checksums
so add a SW one.

Also add a "rx_errors" sysctl with a copy of netstat IERRORS
counter value to make it easier accessible from scripts.

Reviewed By:	erj
Tested By:	gowtham.kumar.ks@intel.com
Sponsored By:	Intel Corporation
Differential Revision:	https://reviews.freebsd.org/D27639

(cherry picked from commit 9f99061ef9c95b171fc92d34026222bb5e052337)