Use __attribute__((malloc)) on memory allocation functions

Use __attribute__((malloc)) on memory allocation functions

This informs the C compiler that pointers returned from these functions
do not alias other functions, which allows it to do better code
optimization and should make the compiled code smaller.

References:
https://stackoverflow.com/a/53654773
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-malloc-function-attribute
https://clang.llvm.org/docs/AttributeReference.html#malloc

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14827