ext4: teach ext4_ext_truncate() about the bigalloc feature

When we are truncating (as opposed unlinking) a file, we need to worry
about partial truncates of a file, especially in the light of sparse
files.  The changes here make sure that arbitrary truncates of sparse
files works correctly.  Yeah, it's messy.

Note that these functions will need to be revisted when the punch
ioctl is integrated --- in fact this commit will probably have merge
conflicts with the punch changes which Allison Henders and the IBM LTC
have been working on.  I will need to fix this up when either patch
hits mainline.

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