tmpfs/ramfs: let memfd_create() work on nommu

Currently trying to use memfd_create() on nommu returns an error with
errno set to EFBIG.  The manpage memfd_create() doesn't have EFBIG as a
possible error value.

Doing some digging this is coming from 0 getting passed as newsize to
ramfs_nommu_expand_for_mapping() and that getting into get_order() and
there "The result is undefined if the size is 0".

Whatever comes out of get_order() is then used in the following logic and
that results in the EFBIG that causes the syscall to fail and the errno in
userspace.

If newsize is 0 there is nothing to do so just return.

Roughly tested on m68k nommu by creating a process, creating an memfd,
forking another process, mmap()ing the memfd in the child, writing into
the mapping, then mmap()ing in the parent and checking that the right data
is there.

Link: https://lore.kernel.org/20260523130445.1101818-1-daniel@thingy.jp
Signed-off-by: Daniel Palmer <daniel@thingy.jp>
Acked-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/ramfs/file-nommu.c

@@ -69,6 +69,9 @@ int ramfs_nommu_expand_for_mapping(struct inode *inode, size_t newsize)
 	gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
 
 	/* make various checks */
+	if (!newsize)
+		return 0;
+
 	order = get_order(newsize);
 	if (unlikely(order > MAX_PAGE_ORDER))
 		return -EFBIG;