binfmt: remove in-tree usage of MAP_DENYWRITE

At exec time when we mmap the new executable via MAP_DENYWRITE we have it
opened via do_open_execat() and already deny_write_access()'ed the file
successfully. Once exec completes, we allow_write_acces(); however,
we set mm->exe_file in begin_new_exec() via set_mm_exe_file() and
also deny_write_access() as long as mm->exe_file remains set. We'll
effectively deny write access to our executable via mm->exe_file
until mm->exe_file is changed -- when the process is removed, on new
exec, or via sys_prctl(PR_SET_MM_MAP/EXE_FILE).

Let's remove all usage of MAP_DENYWRITE, it's no longer necessary for
mm->exe_file.

In case of an elf interpreter, we'll now only deny write access to the file
during exec. This is somewhat okay, because the interpreter behaves
(and sometime is) a shared library; all shared libraries, especially the
ones loaded directly in user space like via dlopen() won't ever be mapped
via MAP_DENYWRITE, because we ignore that from user space completely;
these shared libraries can always be modified while mapped and executed.
Let's only special-case the main executable, denying write access while
being executed by a process. This can be considered a minor user space
visible change.

While this is a cleanup, it also fixes part of a problem reported with
VM_DENYWRITE on overlayfs, as VM_DENYWRITE is effectively unused with
this patch and will be removed next:
  "Overlayfs did not honor positive i_writecount on realfile for
   VM_DENYWRITE mappings." [1]

[1] https://lore.kernel.org/r/YNHXzBgzRrZu1MrD@miu.piliscsaba.redhat.com/

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

fs/binfmt_elf_fdpic.c

@@ -1041,7 +1041,7 @@ static int elf_fdpic_map_file_by_direct_mmap(struct elf_fdpic_params *params,
 		if (phdr->p_flags & PF_W) prot |= PROT_WRITE;
 		if (phdr->p_flags & PF_X) prot |= PROT_EXEC;
 
-		flags = MAP_PRIVATE | MAP_DENYWRITE;
+		flags = MAP_PRIVATE;
 		maddr = 0;
 
 		switch (params->flags & ELF_FDPIC_FLAG_ARRANGEMENT) {