Rename "dn_szsec" field in dnode structures

The "dn_szsec" field in the on disk and in RAM dnode structure was
modified to remove it's association with 512 byte disk sectors.

The on-disk structure now has a "dn_nextra" field that corresponds to
the number of extra subsequent dnode_phys_t slots it consumes within the
dnode block. Thus a regular 512 byte dnode will store "0" in this field.

The in RAM structure now has a "dn_count" field which corresponds to the
total amount of dnode_phys_t slots the on-disk structure consumes. In
contrast to the on-disk structure, a regular 512 byte dnode will store
"1" in this field (since a dnode stored on disk must consume at least
one dnode_phys_t slot in a given dnode block).

The zdb output was also slightly modified to be a little more consistent
with the "dnodesize" filesystem attribute. Now, the size of a dnode will
be output in units of bytes, rather than number of dnode_phys_t slots
consumed within the dnode block.

Signed-off-by: Prakash Surya <surya1@llnl.gov>