Fix corrupted l2_asize in arcstats

Commit e0b0ca9 accidentally corrupted the l2_asize displayed in
arcstats. This was caused by changing the l2arc_buf_hdr.b_asize
member from an int to uint32_t type. There are places in the
code where this field is cast to a uint64_t resulting in the
b_hits member being treated as part of b_asize.

To resolve the issue the type has been changed to a uint64_t,
and the b_hits member is placed after the enum to prevent the
size of the structure from increasing.

This is a good example of exactly why it's a bad idea to use
ambiguous types (int) in these structures.

Signed-off-by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1990