lsm: constify 'sb' parameter in security_quotactl()

SELinux registers the implementation for the "quotactl" hook. Looking at
the function implementation we observe that the parameter "sb" is not
changing.

Mark the "sb" parameter of LSM hook security_quotactl() as "const" since
it will not be changing in the LSM hook.

Signed-off-by: Khadija Kamran <kamrankhadijadj@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>