reiserfs: Fix spurious unlock in reiserfs_fill_super() error handling

When we fail to allocate string for journal device name we jump to
'error' label which tries to unlock reiserfs write lock which is not
held. Jump to 'error_unlocked' instead.

Fixes: f32485be ("reiserfs: delay reiserfs lock until journal initialization")
Signed-off-by: Jan Kara <jack@suse.cz>