Move zap_attribute_t to the heap in dsl_deadlist_merge

In the case of a regular compilation, the compiler
raises a warning for a dsl_deadlist_merge function, that
the stack size is to large. In debug build this can
generate an error.

Move large structures to heap.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #14524