user_namespace: remove unnecessary NULL values from kbuf

kbuf is assigned first, so it does not need to initialize the assignment.

Link: https://lkml.kernel.org/r/20240115062519.31298-1-zeming@nfschina.com


Signed-off-by: Li zeming <zeming@nfschina.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>