[PATCH] bdev: use correct mapping's i_sem

[PATCH] bdev: use correct mapping's i_sem

From: viro@parcelfarce.linux.theplanet.co.uk <viro@parcelfarce.linux.theplanet.co.uk>

In a bunch of places we used file->f_dentry->d_inode->i_sem to protect
fdatasync et.al.  Replaced with corrent file->f_mapping->host->i_sem - the
object we are protecting is address_space, so we want an exclusion that would
work for redirected ->i_mapping.  For normal files (not coda, not bdev) it's
all the same, of course - there we have

 	file->f_mapping->host == file->f_dentry->d_inode

and change above is an equivalent transfromation.