Merge patch series "ovl: file descriptors based layer setup"

Christian Brauner <brauner@kernel.org> says:

Currently overlayfs only allows specifying layers through path names.
This is inconvenient for users such as systemd that want to assemble an
overlayfs mount purely based on file descriptors.

When porting overlayfs to the new mount api I already mentioned this.
This enables user to specify both:

     fsconfig(fd_overlay, FSCONFIG_SET_FD, "upperdir+", NULL, fd_upper);
     fsconfig(fd_overlay, FSCONFIG_SET_FD, "workdir+",  NULL, fd_work);
     fsconfig(fd_overlay, FSCONFIG_SET_FD, "lowerdir+", NULL, fd_lower1);
     fsconfig(fd_overlay, FSCONFIG_SET_FD, "lowerdir+", NULL, fd_lower2);

in addition to:

     fsconfig(fd_overlay, FSCONFIG_SET_STRING, "upperdir+", "/upper",  0);
     fsconfig(fd_overlay, FSCONFIG_SET_STRING, "workdir+",  "/work",   0);
     fsconfig(fd_overlay, FSCONFIG_SET_STRING, "lowerdir+", "/lower1", 0);
     fsconfig(fd_overlay, FSCONFIG_SET_STRING, "lowerdir+", "/lower2", 0);

The selftest contain an example for this.

* patches from https://lore.kernel.org/r/20241014-work-overlayfs-v3-0-32b3fed1286e@kernel.org:
  selftests: add overlayfs fd mounting selftests
  selftests: use shared header
  Documentation,ovl: document new file descriptor based layers
  ovl: specify layers via file descriptors
  fs: add helper to use mount option as path or fd

Link: https://lore.kernel.org/r/20241014-work-overlayfs-v3-0-32b3fed1286e@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>