sys/vfs/hammer: Adjust hammer_debug_general

sys/vfs/hammer: Adjust hammer_debug_general

Change hammer_debug_general in hammer_blockmap_getfree()
to 0x4000 which is the same value as those in
sys/vfs/hammer/hammer_reblock.c.

This is better since hammer_blockmap_getfree() is made
for reblock and only used by reblock (0x0800 picks up
other debug kprintfs that aren't related to reblock).

Also note that hammer_debug_general values are randomly
picked based on use cases rather than fixed value for
each file/etc.

Add some comments on the conditionals that are used to
start data/node reblock since this part is not too easy
to understand.