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
MFC after: 3 days
Fixes: 6eb925f8450f ("Filesystem utilities that modify the...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29021