libmount: improve mnt_reset_fs()

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: reuse allocated fs in parser

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: add private mnt_context_get_mtab_for_target()

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: fix utab entry on remount

mount(8) command does not set ROOT= field to utab entry on remount,
for example:

	mount -oremount,_netdev /mnt

Reported-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>