libmount: improve MS_REC usage

libmount: improve MS_REC usage

libmount allows to split one library (mount(8)) call to multiple mount(2)
syscalls, for example

   --rbind --make-rslave

in this case we have to be careful with MS_REC because the flag is
applied to multiple operations.

 # strace -e mount mount --rbind --make-rslave /mnt/A /mnt/B

Old version:

 mount("/mnt/A", "/mnt/B", 0x13ecac0, MS_MGC_VAL|MS_BIND, NULL) = 0
 mount("none", "/mnt/B", NULL, MS_REC|MS_SLAVE, NULL) = 0

Fixed version:

 mount("/mnt/A", "/mnt/B", 0x1f22ac0, MS_MGC_VAL|MS_BIND|MS_REC, NULL) = 0
 mount("none", "/mnt/B", NULL, MS_REC|MS_SLAVE, NULL) = 0

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1584443
Signed-off-by: Karel Zak <kzak@redhat.com>