fs/orangefs: Replace kzalloc + copy_from_user with memdup_user_nul

fs/orangefs: Replace kzalloc + copy_from_user with memdup_user_nul

Replace kzalloc() followed by copy_from_user() with memdup_user_nul() to
simplify and improve orangefs_debug_write(). Allocate only 'count' bytes
instead of the maximum size ORANGEFS_MAX_DEBUG_STRING_LEN, and set 'buf'
to NULL to ensure kfree(buf) still works.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>