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

include/sys/abd.h

@@ -86,10 +86,15 @@ extern int zfs_abd_scatter_enabled;
  * Allocations and deallocations
  */
 
+__attribute__((malloc))
 abd_t *abd_alloc(size_t, boolean_t);
+__attribute__((malloc))
 abd_t *abd_alloc_linear(size_t, boolean_t);
+__attribute__((malloc))
 abd_t *abd_alloc_gang(void);
+__attribute__((malloc))
 abd_t *abd_alloc_for_io(size_t, boolean_t);
+__attribute__((malloc))
 abd_t *abd_alloc_sametype(abd_t *, size_t);
 boolean_t abd_size_alloc_linear(size_t);
 void abd_gang_add(abd_t *, abd_t *, boolean_t);