udf: Protect all modifications of LVID with s_alloc_mutex

udf_open_lvid() and udf_close_lvid() were modifying LVID without
s_alloc_mutex. Since they can be called from remount, the modification
could race with other filesystem modifications of LVID so protect them
by s_alloc_mutex just to be sure.

Signed-off-by: Jan Kara <jack@suse.cz>