libext2fs: don't error out when punching a totally sparse file

libext2fs: don't error out when punching a totally sparse file

If we're asked to punch a file with no data blocks mapped to it and a
non-zero length, we don't need to do any work in ext2fs_punch_extent()
and can return success.  Unfortunately, the extent_get() function
returns "no current node" because it (correctly) failed to find any
extents, which is bubbled up to callers.  Since no extents being found
is not an error in this corner case, fix up ext2fs_punch_extent() to
return 0 to callers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>