file-systems: Always do recursive bind mounts.

Fixes <https://issues.guix.gnu.org/59185>.

* guix/build/syscalls.scm (MS_REC): New variable.
* gnu/build/file-systems.scm (mount-flags->bit-mask): Set MS_REC bit when
bind-mounting.

gnu/build/file-systems.scm

@@ -1127,7 +1127,7 @@ corresponds to the symbols listed in FLAGS."
       (('read-only rest ...)
        (logior MS_RDONLY (loop rest)))
       (('bind-mount rest ...)
-       (logior MS_BIND (loop rest)))
+       (logior MS_REC (logior MS_BIND (loop rest))))
       (('no-suid rest ...)
        (logior MS_NOSUID (loop rest)))
       (('no-dev rest ...)

guix/build/syscalls.scm

@@ -51,6 +51,7 @@
             MS_RELATIME
             MS_BIND
             MS_MOVE
+            MS_REC
             MS_SHARED
             MS_LAZYTIME
             MNT_FORCE
@@ -541,6 +542,7 @@ the last argument of `mknod'."
 (define MS_NODIRATIME      2048)
 (define MS_BIND            4096)
 (define MS_MOVE            8192)
+(define MS_REC            16384)
 (define MS_SHARED       1048576)
 (define MS_RELATIME     2097152)
 (define MS_STRICTATIME 16777216)