fs: add missing $$(sep) to pre- and post-command hooks code

When multiple hooks are registred, both pre-a and post-command hooks'
foreach loops need to have a separator at the end in order for the
code to work as intended. Without the separator all hooks end up as a
one single line command thus making all but the first hook into
no-ops.

Fixes: 4628b6f3 ("fs: add pre- and post-command hooks")
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>