Staging: pohmelfs/dir.c: Remove unneeded mutex_unlock() from pohmelfs_rename()

I do not see any reason for the mutex_unlock(&inode->i_mutex); in
pohmelfs_rename().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>