This patch adds a security_file_alloc call to init_private_file and
creates a close_private_file function to encapsulate the release of
private file structures.  These changes ensure that security
structures for private files will be allocated and freed
appropriately.  Per Andi Kleen's comments, the patch also renames
init_private_file to open_private_file to force updating of all
callers, since they will also need to be updated to use
close_private_file to avoid a leak of the security structure.  Per
Christoph Hellwig's comments, the patch also replaces the 'mode'
argument with a 'flags' argument, computing the f_mode from the flags,
and it explicitly tests f_op prior to dereferencing, as in
dentry_open().