btrfs-progs: convert: Convert ext inode flags to btrfs inode flags

btrfs-progs: convert: Convert ext inode flags to btrfs inode flags

Before this patch, btrfs-convert never copy ext* inode flags to
corresponding btrfs inode flags.

This makes common flags like APPEND/SYNC/SYNCDIR/IMMUTABLE not copied to
btrfs inode.

This patch introduces ext2_convert_inode_flags() function to handle the
convert, so btrfs-convert can copy as many inode flags as possible.

Reported-by: Lakshmipathi.G <lakshmipathi.g@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>