erofs: use kmemdup_nul in erofs_fill_symlink

Remove open coding in erofs_fill_symlink.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/all/20240425222847.GN2118490@ZenIV


Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
Link: https://lore.kernel.org/r/20240902083147.450558-2-toolmanp@tlmp.cc


Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>