Fix "make install" with DESTDIR set (#16995)

"DESTDIR=/path/to/target/root/ make install" may fail when installing to
a root that contains an existing lib/modules structure. When run as root
we may even affect the wrong kernel (the build system's one, or, if
running a different version, some other directory in /lib/modules, but
not the desired one installed in DESTDIR).

Add a missing reference to the INSTALL_MOD_PATH root when calling
"depmod" during "make install"

Also add a switch "DONT_DELETE_MODULES_FILES=1" that skips the removal
of files named "modules.*" prior to running depmod.

Signed-off-by: Christian Kohlschütter <christian@kohlschutter.com>
Closes #16994

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>