acct_on(): don't mess with freeze protection

What happens there is that we are replacing file->path.mnt of
a file we'd just opened with a clone and we need the write
count contribution to be transferred from original mount to
new one.  That's it.  We do *NOT* want any kind of freeze
protection for the duration of switchover.

IOW, we should just use __mnt_{want,drop}_write() for that
switchover; no need to bother with mnt_{want,drop}_write()
there.

Tested-by: Amir Goldstein <amir73il@gmail.com>
Reported-by:  <syzbot+2a73a6ea9507b7112141@syzkaller.appspotmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>