vfs_unregister: fix error handling

Due to misplaced braces, an error from vfs_uninit() in the VFCF_SBDRY
case was ignored.

Reported by: Anton Rang <rang@acm.org>
Reviewed by: Anton Rang <rang@acm.org>, markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34375