btrfs: avoid multiple i_size rounding in btrfs_truncate()

btrfs: avoid multiple i_size rounding in btrfs_truncate()

We have the inode locked so no one can concurrently change its i_size and
neither do we change it ourselves, so there's no point in keep rounding
it in the while loop and setting it up in the control structure. That only
causes confusion when reading the code.

So move all the i_size setup and rounding out of the loop and assert the
inode is locked.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>