hostfs: fix string handling in __dentry_name()

strcpy() should not be used with destination potentially overlapping
the source; what's more, strscpy() in there is pointless - we already
know the amount we want to copy; might as well use memcpy().

Fixes: c278e81b "hostfs: Remove open coded strcpy()"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>