bpf: have bpf_real_data_inode() take a struct file

bpf: have bpf_real_data_inode() take a struct file

bpf_real_data_inode() must be usable from the bprm_check_security,
mmap_file and file_mprotect hooks for systemd's RestrictFilesystemAccess
BPF LSM program, so have it take a struct file instead of a dentry.

Amir Goldstein <amir73il@gmail.com> suggests:

  While doing so, rename it from bpf_real_inode() to
  bpf_real_data_inode(). For a regular file on a union/overlay
  filesystem it resolves to the underlying inode that hosts the data,
  but for a non-regular file it returns the overlay inode. The new name
  makes the "inode hosting the data" intent explicit and avoids the
  ambiguity of "the real inode backing a file". Document the
  non-regular-file behavior in the kfunc too.

Both the signature change and the rename are safe because the kfunc
landed this cycle and has no released users.

Link: https://patch.msgid.link/20260623-work-bpf-real_inode-v2-1-8e8b57dd25f7@kernel.org
Fixes: 9af8c8a54f6e ("bpf: add bpf_real_inode() kfunc")
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>