cifs: avoid extra allocation for small cifs.idmap keys
The cifs.idmap keytype always allocates memory to hold the payload from userspace. In the common case where we're translating a SID to a UID or GID, we're allocating memory to hold something that's less than or equal to the size of a pointer. When the payload is the same size as a pointer or smaller, just store it in the payload.value union member instead. That saves us an extra allocation on the sid_to_id upcall. Note that we have to take extra care to check the datalen when we go to dereference the .data pointer in the union, but the callers now check that as a matter of course anyway. Reviewed-by: NShirish Pargaonkar <shirishpargaonkar@gmail.com> Signed-off-by: NJeff Layton <jlayton@redhat.com> Signed-off-by: NSteve French <smfrench@gmail.com>
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