libmount: improve EPERM interpretation

libmount: improve EPERM interpretation

In some cases mount(2)/open_tree(2) returns EPERM for root user. In
this case libmount reports it as "mount point is not a directory".

It does not makes sense for bind mount where target could be a
regular file.

This patch is not ideal, the error handler is generic, but semantic
for new mount API and mount(2) is different. For example now it checks
for regular file, but the new API supports bind over symlinks, so
proper fix will require lstat() and S_ISLNK(), etc. We need to move
error messages to hook_mount.c and mount_mount_legacy.c to make it
more specific.

Fixes: https://github.com/util-linux/util-linux/issues/2413
Signed-off-by: Karel Zak <kzak@redhat.com>