lxd/storage/drivers/utils: Passes true for keepBlockDev arg to UnmounTask in shrinkFileSystem
lxd/storage/drivers/utils: Passes true for keepBlockDev arg to UnmounTask in shrinkFileSystem

Allows for a block backed storage backend to keep its block device activate to allow shrink of filesystem if filesystem was already mounted.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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