vt(4): Skip vt_flush() for nested panics

[Why]
If there is a problem with DRM drivers or in their integration with
vt(4) and displaying something on the console triggers a panic, there is
a high chance that displaying that panic will trigger another one,
recursively.

[How]
If vt_flush() is called and it detects is is called resursively from
another panic, it return immediately, doing nothing, to avoid the risk
of triggering another panic.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42056