btrfs: fix mismatching parameter names for btrfs_get_extent()

The definition for btrfs_get_extent() is using "u64 end" as the last
parameter, but in implementation we go "u64 len", and all call sites
follows the implementation.

This can be very confusing during development, as most developers
including me, would just use the snippet returned by LSP (clangd in my
case), which would only check the definition.

Unfortunately this mismatch is introduced from the very beginning of
btrfs.

Fix it to prevent further confusion.

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