fs: support O_PATH fds with FSCONFIG_SET_FD

Let FSCONFIG_SET_FD handle O_PATH file descriptors. This is particularly
useful in the context of overlayfs where layers can be specified via
file descriptors instead of paths. But userspace must currently use
non-O_PATH file desriptors which is often pointless especially if
the file descriptors have been created via open_tree(OPEN_TREE_CLONE).

Link: https://lore.kernel.org/r/20250210-work-overlayfs-v2-1-ed2a949b674b@kernel.org


Fixes: a08557d1 ("ovl: specify layers via file descriptors")
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>