LU-4252 osd: remove locking i_mutex in osd_punch

LU-4252 osd: remove locking i_mutex in osd_punch

This piece of code intent to calm down the kernel
WARN_ON messsage show up since 3.9 upstream. So we
can just remove it for now. We'll add a ldiskfs
patch to resolve this issue while 3.9 server support
come up.

Signed-off-by: yang sheng <yang.sheng@intel.com>
Change-Id: I0d8a13829ebb1d18c672a09729be603c923bc79c
Reviewed-on: http://review.whamcloud.com/9742
Tested-by: Jenkins
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>