ubifs: Reserve one leb for each journal head while doing budget

UBIFS calculates available space by c->main_bytes - c->lst.total_used
(which means non-index lebs' free and dirty space is accounted into
total available), then index lebs and four lebs (one for gc_lnum, one
for deletions, two for journal heads) are deducted.
In following situation, ubifs may get -ENOSPC from make_reservation():
 LEB 84: DATAHD   free 122880 used 1920  dirty 2176  dark 6144
 LEB 110:DELETION free 126976 used 0     dirty 0     dark 6144 (empty)
 LEB 201:gc_lnum  free 126976 used 0     dirty 0     dark 6144
 LEB 272:GCHD     free 77824  used 47672 dirty 1480  dark 6144
 LEB 356:BASEHD   free 0      used 39776 dirty 87200 dark 6144
 OTHERS: index lebs, zero-available non-index lebs

UBIFS calculates the available bytes is 6888 (How to calculate it:
126976 * 5[remain main bytes] - 1920[used] - 47672[used] - 39776[used] -
126976 * 1[deletions] - 126976 * 1[gc_lnum] - 126976 * 2[journal heads]
- 6144 * 5[dark] = 6888) after doing budget, however UBIFS cannot use
BASEHD's dirty space(87200), because UBIFS cannot find next BASEHD to
reclaim current BASEHD. (c->bi.min_idx_lebs equals to c->lst.idx_lebs,
the empty leb won't be found by ubifs_find_free_space(), and dirty index
lebs won't be picked as gced lebs. All non-index lebs has dirty space
less then c->dead_wm, non-index lebs won't be picked as gced lebs
either. So new free lebs won't be produced.). See more details in Link.

To fix it, reserve one leb for each journal head while doing budget.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216562


Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>