linuxkpi: Don't destroy the mutex in xa_destroy()

[Why]
The mutex initialized in xa_init_flags() is not destroyed here on
purpose. The reason is that on Linux, the xarray remains usable after a
call to xa_destroy(). For instance the i915 DRM driver relies on that
during the initialixation of its GuC. Basically, xa_destroy() "resets"
the structure to zero but doesn't really destroy it.

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48762