f2fs: Replace strncpy with memcpy

gcc 8.1.0 complains:

fs/f2fs/namei.c: In function 'f2fs_update_extension_list':
fs/f2fs/namei.c:257:3: warning:
	'strncpy' output truncated before terminating nul copying
	as many bytes from a string as its length
fs/f2fs/namei.c:249:3: warning:
	'strncpy' output truncated before terminating nul copying
	as many bytes from a string as its length

Using strncpy() is indeed less than perfect since the length of data to
be copied has already been determined with strlen(). Replace strncpy()
with memcpy() to address the warning and optimize the code a little.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>