Squashfs: add SEEK_DATA/SEEK_HOLE support

Add support for SEEK_DATA and SEEK_HOLE lseek() whence values.

These allow much faster searches for holes and data in sparse files, which
can significantly speed up file copying, e.g.

before (GNU coreutils, Debian 13):

cp --sparse=always big-file /

took real 11m58s, user 0m5.764s, sys 11m48s

after:

real 0.047s, user 0.000s, sys 0.027s

Where big-file has a 256 GB hole followed by 47 KB of data.

Link: https://lkml.kernel.org/r/20250923220652.568416-3-phillip@squashfs.org.uk


Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>