nilfs2: fix incorrect kernel-doc declaration of nilfs_palloc_req structure

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.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>