ubifs: Fix races between xattr_{set|get} and listxattr operations

ubifs: Fix races between xattr_{set|get} and listxattr operations

UBIFS may occur some problems with concurrent xattr_{set|get} and
listxattr operations, such as assertion failure, memory corruption,
stale xattr value[1].

Fix it by importing a new rw-lock in @ubifs_inode to serilize write
operations on xattr, concurrent read operations are still effective,
just like ext4.

[1] https://lore.kernel.org/linux-mtd/20200630130438.141649-1-houtao1@huawei.com

Fixes: 1e51764a3c2ac05a23 ("UBIFS: add new flash file system")
Cc: stable@vger.kernel.org  # v2.6+
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>