btrfs: use end_pos variable where needed in btrfs_dirty_folio()

btrfs: use end_pos variable where needed in btrfs_dirty_folio()

We have a couple places doing the computation "pos + write_bytes" when we
already have it in the local variable "end_pos". Change then to use the
variable instead and make source code smaller. Also make the variable
const since it's not supposed to change.

This also has a very slight reduction in the module size.

Before:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1915990	 161647	  15592	2093229	 1ff0ad	fs/btrfs/btrfs.ko

After:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1915974	 161647	  15592	2093213	 1ff09d	fs/btrfs/btrfs.ko

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>