gmirror: Fix a bug introduced in r341674

gmirror: Fix a bug introduced in r341674

r341674 inadvertently introduced a bug where newer mirror components being
tasted would clear the high sc_flags that are not controlled by component
metadata, such as G_MIRROR_DEVICE_FLAG_TASTING.  This could plausibly expose
a small window of time during STARTING where device destruction might race
with mirror component addition, probably resulting in a crash.

Reviewed by:	markj
X-MFC-With:	r341674
Differential Revision:	https://reviews.freebsd.org/D18521