make it thread-safe

[osvunikernel]

This patch modifies this fork of lwext4 to make is safe to interact with by multiple threads running in OSv.

The key assumption is, that OSv VFS layer provides necessary locking around all interactions with lwext4 to guard ext filesystem metadata (i-node table, directory entries, etc) modifications confined to specific vnode.

Beyond that, we add necessary locking around 3 key common data structures:

• i-node bitmaps in ext4_ialloc.c
• data block bitmaps in ext4_balloc.c
• metadata blockcache in ext4_bcache.c and related files

More specifically following functions are protected with inode_alloc_lock()/unlock() to make sure no two files/directories get assigned same inode number:

• ext4_ialloc_alloc_inode()
• ext4_ialloc_free_inode()

Next, following functions are protected with block_alloc_lock()/unlock() to make sure no two files/directories use same data block:

• ext4_balloc_alloc_block()
• ext4_balloc_free_block()
• ext4_balloc_free_blocks()

Finally, these functions in ext4_bcache.c and related source files are protected with bcache_lock()/unlock() to make sure the global metadata block cache access is synchronized:

• ext4_bcache_invalidate_lba() in __ext4_balloc_free_block() and __ext4_balloc_free_blocks()
• ext4_bcache_find_get(), ext4_block_flush_buf() and ext4_bcache_free() in ext4_block_flush_lba()
• ext4_block_get_noread(), ext4_bcache_test_flag() ext4_bcache_free() in ext4_block_get()
• ext4_bcache_free() in ext4_block_set()
• ext4_block_get_noread() in ext4_trans_block_get_noread()

Ref gkostka#83
Ref cloudius-systems/osv#1179