btrfs: use a single switch statement when initializing delayed ref head

At init_delayed_ref_head(), we are using two separate if statements to
check the delayed ref head action, and initializing 'must_insert_reserved'
to false twice, once when the variable is declared and once again in an
else branch.

Make this simpler and more straightforward by having a single switch
statement, also moving the comment about a drop action to the
corresponding switch case to make it more clear and eliminating the
duplicated initialization of 'must_insert_reserved' to false.

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