btrfs: fix extent map logging bit not cleared for split maps after dropping range

At btrfs_drop_extent_map_range() we are clearing the EXTENT_FLAG_LOGGING
bit on a 'flags' variable that was not initialized. This makes static
checkers complain about it, so initialize the 'flags' variable before
clearing the bit.

In practice this has no consequences, because EXTENT_FLAG_LOGGING should
not be set when btrfs_drop_extent_map_range() is called, as an fsync locks
the inode in exclusive mode, locks the inode's mmap semaphore in exclusive
mode too and it always flushes all delalloc.

Also add a comment about why we clear EXTENT_FLAG_LOGGING on a copy of the
flags of the split extent map.

Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/Y%2FyipSVozUDEZKow@kili/


Fixes: db21370b ("btrfs: drop extent map range more efficiently")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>