Suppress Clang Static Analyzer false positive in the AVL tree code.

This has been filed as llvm/llvm-project#60694. Switching from a copy
through a C pointer dereference to an explicit memcpy() is a workaround
that prevents a false positive.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14575