e2fsck: reset @inodes_to_rebuild if restart

e2fsck: reset @inodes_to_rebuild if restart

Verify multiple thread on a corrupted images hit following bug:

pass1.c:2902: e2fsck_pass1_thread_prepare:
Assertion `global_ctx->inodes_to_rebuild == NULL' failed.
Signal (6) SIGABRT si_code=SI_TKILL
./e2fsck/e2fsck[0x43829e]
/lib64/libpthread.so.0(+0x14b20)[0x7f3b45135b20]
/lib64/libc.so.6(gsignal+0x145)[0x7f3b44f2c625]
/lib64/libc.so.6(abort+0x12b)[0x7f3b44f158d9]
/lib64/libc.so.6(+0x257a9)[0x7f3b44f157a9]
/lib64/libc.so.6(+0x34a66)[0x7f3b44f24a66]
./e2fsck/e2fsck(e2fsck_pass1+0x1662)[0x423572]
./e2fsck/e2fsck(e2fsck_run+0x5a)[0x41611a]
./e2fsck/e2fsck(main+0x1608)[0x4121b8]
/lib64/libc.so.6(__libc_start_main+0xf3)[0x7f3b44f171a3]
./e2fsck/e2fsck(_start+0x2e)[0x413dde]

@inodes_to_rebuild could be not NULL after we restart pass1

E2fsprogs-commit: 3b7879e310d30d5f4c518c607aec830b63b2315b

Change-Id: I8ca6a37d41788333fe9494bd84816932f39fcb3c
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>