eCryptfs: fix permission denied with ecryptfs_xattr mount option when create readonly file

When the ecryptfs_xattr mount option is turned on, the ecryptfs
metadata will be written to xattr via vfs_setxattr, which will
check the WRITE permissions.

However, this will cause denial of permission when creating a
file withoug write permission.

So fix this by calling __vfs_setxattr directly to skip permission
check.

Signed-off-by: Robbie Ko <robbieko@synology.com>
[tyhicks: Copy up lower inode attributes when successful]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>