Remove unused 'dsl_pool_t *dp' variable

When ASSERTs are compiled out by using the --disable-debug configure
option. Then the local variable 'dsl_pool_t *dp' will be unused and
generate a compiler warning. Since this variable is only used once
in the ASSERT replace it with 'ds->ds_dir->dd_pool'.

This has the additional advantage of potentially saving a few bytes
on the stack depending on how gcc decides to compile the function.

This issue was not noticed immediately because the automated builders
use --enable-debug to make the testing as rigorous as possible.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Closes #3410