posix: implement munmap()

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, munmap() is one of them.

munmap - unmap pages of memory

The munmap() function shall remove any mappings for those entire pages containing any part of the address space of the process starting at addr and continuing for len bytes. Further references to these pages shall result in the generation of a SIGSEGV signal to the process. If there are no mappings in the specified address range, then munmap() has no effect.

The implementation may require that addr be a multiple of the page size as returned by sysconf().

If a mapping to be removed was private, any modifications made in this address range shall be discarded.

[ML|MLR] [Option Start] Any memory locks (see mlock and mlockall) associated with this address range shall be removed, as if by an appropriate call to munlock(). [Option End]

[TYM] [Option Start] If a mapping removed from a typed memory object causes the corresponding address range of the memory pool to be inaccessible by any process in the system except through allocatable mappings (that is, mappings of typed memory objects opened with the POSIX_TYPED_MEM_MAP_ALLOCATABLE flag), then that range of the memory pool shall become deallocated and may become available to satisfy future typed memory allocation requests.

A mapping removed from a typed memory object opened with the POSIX_TYPED_MEM_MAP_ALLOCATABLE flag shall not affect in any way the availability of that typed memory for allocation. [Option End]

The behavior of this function is unspecified if the mapping was not established by a call to mmap().

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/munmap.html

See RFC #51211 for more info.