growfs: allow operation on RW-mounted filesystems

growfs: allow operation on RW-mounted filesystems

growfs supports growing mounted filesystems (writes are temporarily
suspended while the grow happens).  Drop the check for fs_clean == 0
to restore this case.  Leave fs_flags check for FS_UNCLEAN or
FS_NEEDSFSCK which represent the state of the filesystem when it was
mounted, and fsck should be run first if they are set.

PR:		253754
Reviewed by:	mckusick
Fixes:		6eb925f8450f ("Filesystem utilities that modify the...")
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29021

(cherry picked from commit 0dcde5cc12744e5188300711a8829e5e6a9cd0de)