cryptenroll: Fix reading keyfile from socket

cryptenroll: Fix reading keyfile from socket

systemd-cryptenroll uses the READ_FULL_FILE_CONNECT_SOCKET flag when
reading the keyfile to also allow reading it from a socket. But it also
sets the offset to 0, causing an unnecessary seek to the beginning of
the newly opened keyfile and disables socket support again, as these do
not support seeking.

Disable seeking entirely to remove the unneeded seek and restore support
for reading the keyfile from a socket again as with systemd-cryptsetup.