copy-in: for files, only iterate file blocks that are mapped

copy-in: for files, only iterate file blocks that are mapped

Rewrite the file copy-in algorithm to detect smaller holes in the
files we're copying in.  Use SEEK_DATA/SEEK_HOLE/FIEMAP when available
to skip known empty parts.  This fixes the particular bug where zeroed
blocks on a system with 64k pages are needlessly copied into a
4k-block filesystem.  It also saves time by skipping parts we know to
be zeroed.

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