cache the value of file_inode() in struct file

Note that this thing does *not* contribute to inode refcount;
it's pinned down by dentry.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/file_table.c
Simple merge

diff --cc fs/open.c
Simple merge

diff --cc include/linux/fs.h
Simple merge