lib/utils_crypt: optimize seek to keyfile-offset

lib/utils_crypt: optimize seek to keyfile-offset

Avoid using unbuffered reads when "seeking" to a keyfile offset. This is
abysmally slow when the key is hidden at the end of a large device.
Instead, try to actually call lseek, falling back on reading in chunks
of BUFSIZ bytes until the desired offset is reached.

Command line:

  cryptsetup luksOpen /dev/vdc1 home \
      --keyfile /dev/vdd --keyfile-size 4096 --keyfile-offset 123456789

Before:

  real   0m25.589s
  user   0m7.030s
  sys    0m18.479s

After:

  real   0m4.464s
  user   0m4.253s
  sys    0m0.157s