btrfs: start transaction in xattr_handler_set_prop

btrfs specific extended attributes on the inode are set using
btrfs_xattr_handler_set_prop(), and the required transaction for this
update is started by btrfs_setxattr(). For better visibility of the
transaction start and end, do this in btrfs_xattr_handler_set_prop().
For which this patch copied code of btrfs_setxattr() as it is in the
original, which needs proper error handling.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>