nilfs2: describe the members of nilfs_bmap_operations structure

nilfs2: describe the members of nilfs_bmap_operations structure

Add missing member variable descriptions in the kernel-doc comments for
the nilfs_bmap_operations structure, hiding the internal operations with
the "private:" tag.  This eliminates the following warnings output by the
kernel-doc script:

 fs/nilfs2/bmap.h:74: warning: Function parameter or struct member
  'bop_lookup' not described in 'nilfs_bmap_operations'
 fs/nilfs2/bmap.h:74: warning: Function parameter or struct member
  'bop_lookup_contig' not described in 'nilfs_bmap_operations'
 ...
 fs/nilfs2/bmap.h:74: warning: Function parameter or struct member
  'bop_gather_data' not described in 'nilfs_bmap_operations'

Link: https://lkml.kernel.org/r/20240816074319.3253-7-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>