ubifs: make ubifs_[get|set]xattr atomic

This commit make the ubifs_[get|set]xattr protected by ui_mutex.

Originally, there is a possibility that ubifs_getxattr to get
a wrong value.

  P1                                  P2
----------                  	----------
ubifs_getxattr                      ubifs_setxattr
					- kfree()
- memcpy()
					- kmemdup()

Then ubifs_getxattr() would get a non-sense data. To solve this
problem, this commit make the xattr of ubifs_inode updated in
atomic.

CRs-Fixed: 975289
Change-Id: I4b963339a2fa7465528a24d06f9df57a455bdf12
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: ab92a20bce3b4c21927cc1ae514c482f50ad3487
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>