Btrfs: Abstract similar code for btrfs_block_rsv_add{, _noflush}

btrfs_block_rsv_add{, _noflush}() have similar code, so abstract that code.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>