LU-7623 libcfs: Properly map page for vfs_write

LU-7623 libcfs: Properly map page for vfs_write

Apparently it's possible that the page might not be mapped before
vfs_write, so map it. Also forcefully mark it __user to make
sparse happy.

This was part of upstream kernel commit 7ebcb731 that somehow
never made it back even though vfs_write changes did.

Change-Id: I88494eac08c02d9cac2476eeb56be1eb65cfc3bf
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/17787
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>