trivial: s/i_sem /i_mutex/

This patch substitutes i_sem by i_mutex in
Documentation/filesystems/Locking.
The patch also removes a couple of trailing white-spaces.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>