btrfs: drop unused parameter from extent_map_tree_init

the GFP flags are not stored anywhere and all allocations are done via
alloc_extent_map(GFP_NOFS).

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/extent_map.c
Simple merge

diff --cc fs/btrfs/extent_map.h
Simple merge

diff --cc fs/btrfs/inode.c
Simple merge

diff --cc fs/btrfs/volumes.c
Simple merge