Commit 7df4d3465d and others changed
'mount', 'umount', & co. so they would call 'syscall->procedure' at each
call. This change reverts to the previous behavior, where
'syscall->procedure' is called once.
* guix/build/syscalls.scm (mount, umount, reboot, load-linux-module):
Call 'syscall->procedure' only once.