ec: Fix invalid inode lock in ftruncate

The fops 'truncate' and 'ftruncate' share some code and inodelk()
was always made against the inode inside the loc_t structure
instead of that of fd_t. Since ftruncate has the loc initialized
to NULL, this fop was executed without any lock, allowing some
concurrent modifications in the file size.

Also changed the way in which 'fop' and 'ffop' are differentiated
in shared code. Now it uses 'id' field instead of checking if 'fd'
is NULL.

This is a backport of http://review.gluster.org/8695/

Change-Id: Ibd18accf2652193b395a841b9029729e5f4867c6
BUG: 1140847
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8780
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>