libmount: improve fs referencing in tables

libmount: improve fs referencing in tables

* Added member 'struct libmnt_table *tab' to libmnt_fs structure.
* Added 'mnt_fs_get_table()'.
* Removed overhead from 'mnt_table_{insert,move,remove}_fs().
* Added check to 'mnt_table_set_iter()' that entry is member of table.

[kzak@redhat.com: - add to libmount.sys
                  - add to docs
                  - cleanup commit message
                  - set fs->tab = NULL before mnt_unref_fs() in mnt_table_remove_fs()]

Signed-off-by: Tim Hildering <hilderingt@posteo.net>
Signed-off-by: Karel Zak <kzak@redhat.com>