MFV r267566:

4390 i/o errors when deleting filesystem/zvol can lead to space map corruption

MFC after:	2 weeks
Fix order of USB serial layer uninit. Currently module dependency
rules prevent the USB serial module to be unloaded before any client
modules. This patch ensures that the "ucom_mtx" mutex is destroyed
last when doing a system uninit in a monotolith build aswell.

MFC after:	3 days