btrfs: move open coded extent map tree deletion out of inode eviction

Move the loop that removes all the extent maps from the inode's extent
map tree during inode eviction out of inode.c and into extent_map.c, to
btrfs_drop_extent_map_range(). Anything manipulating extent maps or the
extent map tree should be in extent_map.c.

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