nilfs2: fix incorrect kernel-doc declaration of nilfs_palloc_req structure

The "struct" keyword is missing from the kernel-doc comment of the
nilfs_palloc_req structure, so add it to eliminate the following warning
output by the kernel-doc script:

 fs/nilfs2/alloc.h:46: warning: cannot understand function prototype:
  'struct nilfs_palloc_req '

Link: https://lkml.kernel.org/r/20240816074319.3253-5-konishi.ryusuke@gmail.comSigned-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/alloc.h

@@ -37,7 +37,7 @@ void *nilfs_palloc_block_get_entry(const struct inode *, __u64,
 int nilfs_palloc_count_max_entries(struct inode *, u64, u64 *);
 
 /**
- * nilfs_palloc_req - persistent allocator request and reply
+ * struct nilfs_palloc_req - persistent allocator request and reply
  * @pr_entry_nr: entry number (vblocknr or inode number)
  * @pr_desc_bh: buffer head of the buffer containing block group descriptors
  * @pr_bitmap_bh: buffer head of the buffer containing a block group bitmap