Data pages in gcdat metadata file (i.e. the secondary DAT for GC), are
cleared or even moved back to the normal DAT when a shot of garbage
collection was done.

Buffer heads held by the palloc cache of gcdat must be cleared before
these page cache manipulation.  This adds nilfs_palloc_clear_cache()
to ensure this.
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>