btrfs: don't BUG_ON btrfs_alloc_path() errors

This patch fixes many callers of btrfs_alloc_path() which BUG_ON allocation
failure. All the sites that are fixed in this patch were checked by me to
be fairly trivial to fix because of at least one of two criteria:

 - Callers of the function catch errors from it already so bubbling the
   error up will be handled.
 - Callers of the function might BUG_ON any nonzero return code in which
   case there is no behavior changed (but we still got to remove a BUG_ON)

The following functions were updated:

btrfs_lookup_extent, alloc_reserved_tree_block, btrfs_remove_block_group,
btrfs_lookup_csums_range, btrfs_csum_file_blocks, btrfs_mark_extent_written,
btrfs_inode_by_name, btrfs_new_inode, btrfs_symlink,
insert_reserved_file_extent, and run_delalloc_nocow

Signed-off-by: Mark Fasheh <mfasheh@suse.com>

fs/btrfs/file.c

@@ -855,7 +855,8 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
 	btrfs_drop_extent_cache(inode, start, end - 1, 0);
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
 again:
 	recow = 0;
 	split = start;