reiserfs: Protect reiserfs_quota_write() with write lock

commit 361d94a338a3fd0cee6a4ea32bbc427ba228e628 upstream.

Calls into reiserfs journalling code and reiserfs_get_block() need to
be protected with write lock. We remove write lock around calls to high
level quota code in the next patch so these paths would suddently become
unprotected.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>