fs: add anon_inode_getfile_secure() similar to anon_inode_getfd_secure()

Extending the secure anonymous inode support to other subsystems
requires that we have a secure anon_inode_getfile() variant in
addition to the existing secure anon_inode_getfd() variant.

Thankfully we can reuse the existing __anon_inode_getfile() function
and just wrap it with the proper arguments.

Acked-by: Mickaël Salaün <mic@linux.microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>