keyring-util: Use reported key size to resize buf

keyring-util: Use reported key size to resize buf

According to keyctl(2), the return value for KEYCTL_READ is:

    The amount of data that is available in the key,
    irrespective of the provided buffer size

So, we could pass in a NULL buffer to query the size, then allocate the
exact right amount of space, then call keyctl again to get the key data.
However, we must still keep the for loop to avoid TOCTOU issues: the key
might have been replaced with something bigger while we're busy
allocating the buffer to store it.

Thus, we can actually save a syscall by picking some reasonable default
buffer size and skipping the NULL call to keyctl. If our default is big
enough, we're done and have saved a syscall! If not, then the first call
behaves essentially the same as the NULL call, and we use the size it
returns to reallocate the buffer appropriately.