nilfs2: get rid of nilfs_mapping_init()

nilfs2: get rid of nilfs_mapping_init()

After applying the lockdep warning fixes, nilfs_mapping_init() is no
longer used, so delete it.

Link: https://lkml.kernel.org/r/1647867427-30498-4-git-send-email-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hao Sun <sunhao.th@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>