btrfs: avoid pointless extent map tree search when flushing delalloc

When flushing delalloc, in COW mode at cow_file_range(), before entering
the loop that allocates extents and creates ordered extents, we do a call
to btrfs_drop_extent_map_range() for the whole range. This is pointless
because in the loop we call create_io_em(), which will also call
btrfs_drop_extent_map_range() before inserting the new extent map.

So remove that call at cow_file_range() not only because it is not needed,
but also because it will make the btrfs_drop_extent_map_range() calls made
from create_io_em() waste time searching the extent map tree, and that
tree can be large for files with many extents. It also makes us waste time
at btrfs_drop_extent_map_range() allocating and freeing the split extent
maps for nothing.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>