ext4: move common truncate functions to header file

Move two functions that will be needed by the indirect functions to be
moved to indirect.c as well as inode.c to truncate.h as inline
functions, so that we can avoid having duplicate copies of the
function (which can be a maintenance problem) without having to expose
them as globally functions.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>