erofs: clean up erofs_register_sysfs()

After commit 684b290a ("erofs: add support for
FS_IOC_GETFSSYSFSPATH"), `sb->s_sysfs_name` is now valid.

Just use it to get rid of duplicated logic.

Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240828095232.571946-1-hsiangkao@linux.alibaba.com