fs/filesystems: Fix potential unsigned integer underflow in fs_name()

fs/filesystems: Fix potential unsigned integer underflow in fs_name()

[ Upstream commit 1363c134ade81e425873b410566e957fecebb261 ]

fs_name() has @index as unsigned int, so there is underflow risk for
operation '@index--'.

Fix by breaking the for loop when '@index == 0' which is also more proper
than '@index <= 0' for unsigned integer comparison.

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/20250410-fix_fs-v1-1-7c14ccc8ebaa@quicinc.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>