Add btrfs_get_extent() and btrfs_punch_hole() for btrfs-progs.
Btrfs_get_extent() will act much like kernel one, return the first
extent that covers the given range.
The difference will be that progs btrfs_get_extent() can't handle
no-holes feature, which means caller should handle it carefully.
Btrfs_punch_hole() will punch a hole in given range of given inode,
however it differs from kernel one since it won't zero any page or drop
any extents if there is any extent in the hole range.
These functions are mainly used for later I_ERR_FILE_EXTENT_DISCOUNT
Signed-off-by: Qu Wenruo <email@example.com>
Signed-off-by: David Sterba <firstname.lastname@example.org>