Fix assertion of free a wrong heap

[royfengsss]

Summary

Fix an assertion issue #9599.

lib_malloc is defined as macro and it will be expanded to kmm_alloc if __KERNEL__ is defined or malloc if __KERNEL__ is not defined.
lib_free is defined as macro and it will be expanded to kmm_free if __KERNEL__ is defined or free if __KERNEL__ is not defined.

For fs, it is a kernel module and build with __KERNEL__ defined. so the lib_free in this module is kmm_free in fact.
For libc is is only a library (not belong to kernel), it build without __KERNEL__ defined. so lib_malloc used in this modue is malloc in fact.

Think about a case like this:
in fs module, asprintf was used to alloc and initialize the memory, and this memory is freed via lib_free. This is the case #9599 addressed.

The fix use the flag of flat build mode. if under flat mode, the __KERNEL__ will not be consider. as it is not real kernel or user distinguished under this buils mode

Impact

Should no

Testing

The issue #9599 has been fixed.

[patacongo]

LGTM

This does interact with CONFIG_MM_KERNEL_HEAP https://github.com/apache/nuttx/blob/master/include/nuttx/kmalloc.h#L84 but that should be fine because that should never be select in the FLAT build.