9p: ->evict_inode() should kick out ->i_data, not ->i_mapping

For block devices the pagecache is associated with the inode
on bdevfs, not with the aliasing ones on the mountable filesystems.
The latter have its own ->i_data empty and ->i_mapping pointing
to the (unique per major/minor) bdevfs inode.  That guarantees
cache coherence between all block device inodes with the same
device number.

Eviction of an alias inode has no business trying to evict the
pages belonging to bdevfs one; moreover, ->i_mapping is only
safe to access when the thing is opened.  At the time of
->evict_inode() the victim is definitely *not* opened.  We are
about to kill the address space embedded into struct inode
(inode->i_data) and that's what we need to empty of any pages.

9p instance tries to empty inode->i_mapping instead, which is
both unsafe and bogus - if we have several device nodes with
the same device number in different places, closing one of them
should not try to empty the (shared) page cache.

Fortunately, other instances in the tree are OK; they are
evicting from &inode->i_data instead, as 9p one should.

Cc: stable@vger.kernel.org # v2.6.32+, ones prior to 2.6.36 need only half of that
Reported-by: "Suzuki K. Poulose" <Suzuki.Poulose@arm.com>
Tested-by: "Suzuki K. Poulose" <Suzuki.Poulose@arm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>