useradd: fix btrfs subvolume creation for single-component basedir

useradd: fix btrfs subvolume creation for single-component basedir

dirname() replaces broken stpcpy index arithmetic that produced an
empty string for single-component paths (e.g. /koolhome), causing
statfs to fail and fall back to a regular directory. Use path in
the error message since dirname() modifies btrfs_check in-place,
making it unusable for logging after the call.

Fixes: c1d36a8acb1d (2019-05-04; "Add support for btrfs subvolumes for user homes")
Signed-off-by: Hadi Chokr <hadichokr@icloud.com>
Reviewed-by: Alejandro Colomar <alx@kernel.org>