fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

In the Linux kernel, the following vulnerability has been resolved:

fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create

anonymous inodes with proper security context. This replaces the current

pattern of calling alloc_anon_inode() followed by

inode_init_security_anon() for creating security context manually.

This change also fixes a security regression in secretmem where the

S_PRIVATE flag was not cleared after alloc_anon_inode(), causing

LSM/SELinux checks to be bypassed for secretmem file descriptors.

As guest_memfd currently resides in the KVM module, we need to export this

symbol for use outside the core kernel. In the future, guest_memfd might be

moved to core-mm, at which point the symbols no longer would have to be

exported. When/if that happens is still unclear.