ubi: fastmap: Don't reserve beb_rsvd_pebs while filling fm pool

hulk inclusion
category: bugfix
bugzilla: 186690, https://gitee.com/openeuler/kernel/issues/I56U8P



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Following script will cause a ENOSPC error while creating ubi volume:
  ID="0x20,0xa5,0x00,0x16"  # 2G-size 128KB-PEB
  modprobe nandsim id_bytes=$ID
  modprobe ubi
  ubiattach -m0
  ubimkvol /dev/ubi0 -N test -s 400MiB # ENOSPC returned

Commit c4d51010 ("ubi: fastmap: Fix high cpu usage of ubi_bgt by
making sure wl_pool not empty") fills free PEBs into fm pool after
reserving 'beb_rsvd_pebs', which will cause that there maybe no enough
free PEBs filled into fm pool (Especially when the UBI device has just
been initialized). Then, ubimkvol could get ENOSPC after ubi attached.
Fix it by filling fm pool without reserving 'beb_rsvd_pebs'.

Fixes: c4d51010 ("ubi: fastmap: Fix high cpu ... not empty")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>