create_inode: handle hard link inum mappings per populate_fs invocation

create_inode: handle hard link inum mappings per populate_fs invocation

When calling populate_fs, the map for hardlink detection is not
cleaned up between populate_fs invocations, which could lead to
unexpected results if anyone calls populate_fs twice in the same
client program).  This doesn't happen right now, but we might as well
clean it up.

The detctor fails if the external directory crosses mountpoints,
so fix that too.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>