qga: ignore EBUSY when freezing a filesystem

the current implementation fails if we try to freeze an
already frozen filesystem. This can happen if a filesystem
is mounted more than once (e.g. with a bind mount).
Suggested-by: NChristian Theune <ct@flyingcircus.io>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit ce2eb6c4)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -1243,6 +1243,9 @@ int64_t qmp_guest_fsfreeze_freeze_list(bool has_mountpoints,
          * filesystems may not implement fsfreeze for less obvious reasons.
          * these will report EOPNOTSUPP. we simply ignore these when tallying
          * the number of frozen filesystems.
+         * if a filesystem is mounted more than once (aka bind mount) a
+         * consecutive attempt to freeze an already frozen filesystem will
+         * return EBUSY.
          *
          * any other error means a failure to freeze a filesystem we
          * expect to be freezable, so return an error in those cases
@@ -1250,7 +1253,7 @@ int64_t qmp_guest_fsfreeze_freeze_list(bool has_mountpoints,
          */
         ret = ioctl(fd, FIFREEZE);
         if (ret == -1) {
-            if (errno != EOPNOTSUPP) {
+            if (errno != EOPNOTSUPP && errno != EBUSY) {
                 error_setg_errno(errp, errno, "failed to freeze %s",
                                  mount->dirname);
                 close(fd);