ubi: fastmap: Use the bitmap API to allocate bitmaps

mainline inclusion
from mainline-v6.1-rc1
commit e7f35da2
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I9195H
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e7f35da21f6f8c6a8c7d262dd4e4bd32e3083f79



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Use bitmap_zalloc()/bitmap_free() instead of hand-writing them.

It is less verbose and it improves the semantic.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>