fs: add LSM-supporting anon-inode interface

This change adds a new function, anon_inode_getfd_secure, that creates
anonymous-node file with individual non-S_PRIVATE inode to which security
modules can apply policy. Existing callers continue using the original
singleton-inode kind of anonymous-inode file. We can transition anonymous
inode users to the new kind of anonymous inode in individual patches for
the sake of bisection and review.

The new function accepts an optional context_inode parameter that callers
can use to provide additional contextual information to security modules.
For example, in case of userfaultfd, the created inode is a 'logical child'
of the context_inode (userfaultfd inode of the parent process) in the sense
that it provides the security context required during creation of the child
process' userfaultfd inode.

Signed-off-by: Daniel Colascione <dancol@google.com>
[LG: Delete obsolete comments to alloc_anon_inode()]
[LG: Add context_inode description in comments to anon_inode_getfd_secure()]
[LG: Remove definition of anon_inode_getfile_secure() as there are no callers]
[LG: Make __anon_inode_getfile() static]
[LG: Use correct error cast in __anon_inode_getfile()]
[LG: Fix error handling in __anon_inode_getfile()]
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>