reiserfs: Protect reiserfs_quota_on() with write lock

commit b9e06ef2e8706fe669b51f4364e3aeed58639eb2 upstream.

In reiserfs_quota_on() we do quite some work - for example unpacking
tail of a quota file. Thus we have to hold write lock until a moment
we call back into the quota code.

Signed-off-by: Jan Kara <jack@suse.cz>
[bwh: Backported to 3.2: there is no distinction between USRQUOTA and
 GRPQUOTA mount options here]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>