Fix the L2ARC write size calculating logic (2)

While commit bcd5321 adjusts the write size based on the size of the log
block, this happens after comparing the unadjusted write size to the
evicted (target) size.

In this case l2ad_hand will exceed l2ad_evict and violate an assertion
at the end of l2arc_write_buffers().

Fix this by adding the max log block size to the allocated size of the
buffer to be committed before comparing the result to the target
size.

Also reset the l2arc_trim_ahead ZFS module variable when the adjusted
write size exceeds the size of the L2ARC device.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14936
Closes #14954