LU-18088 osd: use __vfs_getxattr

LU-18088 osd: use __vfs_getxattr

vfs_getxattr() is not suitable for osd. To get security xattr it goes
via:

vfs_getxattr
  xattr_getsecurity
    security_inode_getsecurity
      cap_inode_getsecurity
        dentry = d_find_any_alias(inode)
          if (!dentry)
            return -EINVAL;
and return -EINVAL because osd object inodes have empty alias list.

Use __vfs_getxattr() instead.

Fixes: b9ef5d1e7f7 ("LU-17546 osd: use __vfs_removexattr")
HPE-bug-id: LUS-12018
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: I71ed0141ebf5124d4ff0862c24fd719f29d883ae
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55917
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>