linux_compat: move kmemdup() from ubifs.c to linux_compat.c

linux_compat.c is the best place for kmemdup(), which is currenly used
only in ubifs.c, but will also be used when other kernel files
(in my case, lib/crypto/x509_cert_parser.c and pkcs7_parser.c) will be
imported. So just move it.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>