nilfs2: add missing description of nilfs_btree_path structure

nilfs2: add missing description of nilfs_btree_path structure

Add missing kernel-doc comment for the 'bp_ctxt' member variable of the
nilfs_btree_path structure, and eliminate the following warning output by
the kenrel-doc script:

 fs/nilfs2/btree.h:39: warning: Function parameter or struct member
  'bp_ctxt' not described in 'nilfs_btree_path'

Link: https://lkml.kernel.org/r/20240816074319.3253-6-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>