ovl: specify layers via file descriptors

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.

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);

Link: https://lore.kernel.org/r/20241014-work-overlayfs-v3-2-32b3fed1286e@kernel.org


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>