Since r273264 the SD card detection on Raspberry Pi is reliably working and
that expose new bugs with HS mode.

When the old code could not do the proper card detection it would boot with
lower defaults (and no HS mode) and this makes some HS cards boots.

Now, with the card always identified as HS capable, the sdhci controller
tries to run the card at HS speeds and makes the boot always fail.

Disable the HS mode for now (which still can be enabled with the tunable)
until it is properly fixed.

MFC with:	r273264
Requested by:	many
SEEK_DATA has interesting behaviour for sparse files on ZFS.  A sparse file
with 128K of random data and truncated to 800K can have SEEK_DATA return -1
when given an offset of 128K. On UFS, the SEEK_DATA returns 800K (the size
of the file). SEEK_HOLE on ZFS seems to behave the same as UFS.

To handle this, map -1 to the size of the file (`end') when lseek returns
this for either SEEK_HOLE or SEEK_DATA. When sparse files are not supported
by the file system both `hole' and `data' will now be equal to `end' and we
will treat the entire file as data. This way, the -1 return for SEEK_DATA
on ZFS will end up doing the right thing.

Reported by: gjb@

MFC after:	3 days