f2fs: clean up xattr operation

1. don't allocate redundant memory in read_all_xattrs.
2. introduce RESERVED_XATTR_SIZE for cleanup.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>