fs: Use fs_mount_t.node to check if system is mounted

The commit changes fs_mount and fs_unmount to use node fs_mount_t
member to decide whether file system described by given fs_mount_t
object has already been mounted.
Previously there was no such check in case of fs_mount, which
would allow to remount the same object as long as mount path
has been changed.
The fs_unmount has been checking whether API pointer (fs) has been
filled now it checks whether fs_mount_t is linked anywhere,
which is sign that the object is used.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>