btrfs: initialize last_extent_end to fix -Wmaybe-uninitialized warning in extent_fiemap()

There's a warning (probably on some older compiler version):

fs/btrfs/fiemap.c: warning: 'last_extent_end' may be used uninitialized in this function [-Wmaybe-uninitialized]:  => 822:19

Initialize the variable to 0 although it's not necessary as it's either
properly set or not used after an error. The called function is in the
same file so this is a false alert but we want to fix all
-Wmaybe-uninitialized reports.

Link: https://lore.kernel.org/all/20240819070639.2558629-1-geert@linux-m68k.org/Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/fiemap.c

@@ -637,7 +637,7 @@ static int extent_fiemap(struct btrfs_inode *inode,
 	struct btrfs_path *path;
 	struct fiemap_cache cache = { 0 };
 	struct btrfs_backref_share_check_ctx *backref_ctx;
-	u64 last_extent_end;
+	u64 last_extent_end = 0;
 	u64 prev_extent_end;
 	u64 range_start;
 	u64 range_end;