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

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>