ocfs2: adjust minlen with discard_granularity in the FITRIM ioctl

Adjust minlen with discard_granularity for FITRIM ioctl(2) if the given
minimum size in bytes is less than it because, discard granularity is
used to tell us that the minimum size of extent that can be discarded by
the storage device.

This is inspired by ext4 commit 5c2ed62f

 ("ext4: Adjust minlen with
discard_granularity in the FITRIM ioctl") from Lukas Czerner.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>