quota: Check presence of quota operation structures instead of ->quota_read and ...

quota: Check presence of quota operation structures instead of ->quota_read and ->quota_write callbacks

Currently we check whether superblock has ->quota_read and ->quota_write
operations to check whether filesystem supports quotas. However for
example for shmfs we will not read or write dquots so check whether
quota operations are set in the superblock instead.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Message-Id: <20230725144510.253763-4-cem@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>