Commit ec9cc18f ("bcachefs: Add checks for invalid snapshot IDs")
intends to check the sanity of a snapshot and panic when
BCACHEFS_DEBUG is set, but that conditional has a typo.

Fix the typo to refer to the actual existing Kconfig symbol.

This was found with ./scripts/checkkconfigsymbols.py.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>