useradd(8): fallback to regular dir for BTRFS home on non-BTRFS parent

useradd(8): fallback to regular dir for BTRFS home on non-BTRFS parent

When the --btrfs-subvolume-home option is used but the parent directory
is not on a BTRFS filesystem, useradd previously failed with an error.
This is too strict; instead, fall back to creating a regular directory
and issue a warning.  The subvolume creation is attempted only when the
parent is BTRFS.  Otherwise, a regular directory is created and a
syslog(3) warning is logged.

Fixes: 3e8c105 (2026-01-02; "src/useradd: Support config for creating home dirs as Btrfs subvolumes")
Co-authored-by: Hadi Chokr <hadichokr@icloud.com>
Co-authored-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>