libmount: use MNT_ERR_APPLYFLAGS for failed mount_setattr()

libmount: use MNT_ERR_APPLYFLAGS for failed mount_setattr()

The classic mount(2) based version uses MNT_ERR_APPLYFLAGS when not
able to set flags and errno is EINVAL. This is expected by
mnt_context_get_mount_excode() where libmount generates error messages
for mount(8).

Expected is:

 # mount --make-private /mnt/test
 mount: /mnt/test: not mount point or bad option.

rather than generic error:

 mount: /mnt/test: wrong fs type, bad option, bad superblock on none ...

Signed-off-by: Karel Zak <kzak@redhat.com>