udf: merge the pieces inserting a new non-directory object into directory

boilerplate code in udf_{create,mknod,symlink} taken to new helper

symlink case converted to unique id calculated by udf_new_inode() - no
point finding a new one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>