nilfs2: move nilfs_bmap_write call out of nilfs_write_inode_common

nilfs2: move nilfs_bmap_write call out of nilfs_write_inode_common

Before converting the disk inode management metadata file ifile, the call
to nilfs_bmap_write(), the i_device_code setting, and the zero-fill code
for inodes on the super root block are moved from
nilfs_write_inode_common() to its callers.

This cleanup simplifies the role and arguments of
nilfs_write_inode_common() and collects calls to nilfs_bmap_write() to the
log writing code.

Also, add and use a new helper nilfs_write_root_mdt_inode() to avoid code
duplication in the data export routine nilfs_segctor_fill_in_super_root()
to the super root block's buffer.

Link: https://lkml.kernel.org/r/20240122140202.6950-9-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>