PROT_WRITE isn't need for a mapping of a file with MAP_SHARED

A file destriptor is opened for read-only and mmap with PROT_WRITE fails.
We don't need PROT_WRITE for this case, because a file contains up to date
data.

lr-x------ 1 root root 64 Dec  1 19:10 20 -> /usr/lib64/gconv/gconv-modules.cache
pos:    0
flags:    0100000
41155 mmap(0x7f2c3d6cf000, 28672, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, 20, 0 <unfinished ...>
41155 <... mmap resumed> )              = -1 EACCES (Permission denied)

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>