befs: Define usercopy region in befs_inode_cache slab cache

befs symlink pathnames, stored in struct befs_inode_info.i_data.symlink
and therefore contained in the befs_inode_cache slab cache, need to be
copied to/from userspace.

cache object allocation:
    fs/befs/linuxvfs.c:
        befs_alloc_inode(...):
            ...
            bi = kmem_cache_alloc(befs_inode_cachep, GFP_KERNEL);
            ...
            return &bi->vfs_inode;

        befs_iget(...):
            ...
            strlcpy(befs_ino->i_data.symlink, raw_inode->data.symlink,
                    BEFS_SYMLINK_LEN);
            ...
            inode->i_link = befs_ino->i_data.symlink;

example usage trace:
    readlink_copy+0x43/0x70
    vfs_readlink+0x62/0x110
    SyS_readlinkat+0x100/0x130

    fs/namei.c:
        readlink_copy(..., link):
            ...
            copy_to_user(..., link, len);

        (inlined in vfs_readlink)
        generic_readlink(dentry, ...):
            struct inode *inode = d_inode(dentry);
            c...