btrfs: simplify early error checking in btrfs_page_mkwrite()

We have this entangled error checks early at btrfs_page_mkwrite():

1) Try to reserve delalloc space by calling btrfs_delalloc_reserve_space()
   and storing the return value in the ret2 variable;

2) If the reservation succeed, call file_update_time() and store the
   return value in ret2 and also set the local variable 'reserved' to
   true (1);

3) Then do an error check on ret2 to see if any of the previous calls
   failed and if so, jump either to the 'out' label or to the
   'out_noreserve' label, depending on whether 'reserved' is true or
   not.

This is unnecessarily complex. Instead change this to a simpler and
more straightforward approach:

1) Call btrfs_delalloc_reserve_space(), if that returns an error jump to
   the 'out_noreserve' label;

2) The call file_update_time() and if that returns an error jump to the
   'out' label.

Like this there's less nested if statements, no need to use a local
variable to track if space was reserved and if statements are used only
to check errors.

Also move the call to extent_changeset_free() out of the 'out_noreserve'
label and under the 'out' label  since the changeset is allocated only if
the call to reserve delalloc space succeeded.

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