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This adds an "fstrim dodtrim" command which will attempt to use
the FIDTRIM ioctl() if FIDTRIM is defined in the uapi/linux/fs.h.
If not, it will fail with EINVAL.

If no FIDTRIM is defined, it does not affect the "fstrim dotrim".

Change-Id: Icbfd621abb592b170db06f749e5eedfd1175f723