btrfs: rename some variables at try_release_extent_mapping()

Rename the following variables:

1) "btrfs_inode" to "inode", because it's shorter to type and clear, and
   we don't have a VFS inode here as well, so there's no confusion;

2) "tree" to "io_tree", to be clear which tree we are dealing with, since
   we use 2 different trees in the function;

3) "map" to "extent_tree" since "map" gives the idea we are dealing with
   an extent map for example, but we are dealing with the inode's extent
   tree (the tree which stores extent maps).

These also make the next patches simpler.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>